Redman saves the day
This one must be done with under 30 painings, as it gets overwritten by other leave endings. Have Paper Hero in your party and talk to Robot Coffin in Rockworld to get Beanie's chairs back. Beanie is located just north of Royale's house. Visit Beanie after getting his chairs back and he'll give you the Decoy Redman. Take the Decoy Redman out on the very first screen of the game and leave to get this special ending.
Job Well Done - Bless all 30 canvases in the Museum
Self-explanatory. None of the canvases are very hard to find if you explore each map well. The last canvases are found as late as right outside where you fight Fabrizia for a second time. Blessings carry over between playthoughs, so don't stress too much if you end up missing one or two your first time around.
Disrespect the Craft - End the game without Blessing a single painting.
Make sure your blessing count stays at 0. Beat all three large enemy crafts and Fabrizia once, then talk to the new NPC on the screen you started the game on to leave.
Morning Monitor - Monitoring's easier than you thought. Collect all 30 Blessings in the Morning and end your day.
Because of one of the last canvases being in the area where you fight Fabrizia, this is impossible to get on a first playthrough. On a repeat playthrough, get 30 blessings and talk to the NPC on the first screen of the game to leave.
Afternoon in the Park - That wasn't so bad at all. Collect all 30 Blessings by Midday and end your day.
Same as Morning Monitor, but beat one of the large enemy crafts to advance the time before you leave.
Lovely Evening - Hard work pays off, now for an evening stroll. Collect all 30 blessings by Evening and end your day.
Same as Morning Monitor and Afternoon in the Park, but beat two of the large enemy crafts instead of one.
Good Night - No need for a party, I want to go home. Collect all 30 Blessings by Night and end your day.
Same as morning Monitor, Afternoon in the Park, and Lovely Evening, but beat all three of the large enemy crafts.
Angel's Train - Monitoring's Harder than you thought. End your day at Dawn.
This one should be possible on a first playthrough. (The reason i say SHOULD is because you are barred entry to the room with the final canvas and second Fabrizia fight on a playthrough where you have not blessed any canvases. I have no clue what the minimum to enter this door is. If it's 30, this must be achieved on a repeat playthrough. Let me know if you know!) Beat Fabrizia a first time, bless the canvas right outside where you fight her a second time, and then leave without fighting her.
I'm Moving Out - The Art World isn't for everyone. Defeat Fabrizia and say goodbye to the print studio.
Beat Fabrizia a second time and the game will automatically end.
Champion of the Underground - Conquer the Freeport's Purple Chip Challenge and become the Champion of the Underground art scene!
In the lower level of the Gamut, beat the Red Chip Challenge, Blue chip Challenge, and Purple Chip Challenge. Tip: Going to Rockworld and talking to Egg there will net you a free Red Chip. I haven't tested if this is only happens at a certain time of day or after you've gone into the Gamut, though.
Those are (to my knowledge) all of the achievements that trigger upon completing the game.

Another Day... - I want to do it allll over again.
Start a New Game+ run.
Print Studio Friends - You get special help from the original Print Studio Friends! They are used to this kind of thing, aren't they? They'll be good help to you.
After giving all 30 blessings, go back to the first screen of the game to find the party members from the first game waiting to help you!
Save Screen Gacha Master - See every Save Screen loading image.
Upon interacting with an elevator to save your game, you are shown a random image. Save enough times and you're bound to see all of them.
Angel vs Angel - Rocky is a very mean person who tried to take me hostage back to the Heavens. I don't want to go up there at all! Sorry for hurting you, but the Canvas is awesome.
Go to Rocky at night to trigger a fight with him. He drops a VERY POWERFUL weapon for Royale.
I Was Sent Back to Level 1?! - I'm Celeste. I was taken to an art museum while living in a Print Studio my older brother tried to conquer. In this musem, I'm "Monitor", and they decayed me--all because I'm an Angel! Luckily, I've got an ability called Observe, which lets me read into the minds of enemies I catch my sights on. With the help of a sleepy young Doctor and a tough Ranger, my journey to make it to the top awaits! I'll prove to everyone that I'm not an Angel--I'm a Bada**!!!
Find Decayman in the Garden (He's in one of the rooms on the left side of the first few screens) and let him reset your level.
The Ultimate Trade - Wait, she gave me a lot of stuff...was that thing really worth that much? She sure does love her cards... Trade the Surrealist Card to Trader.
Buy the Surrealist Card in the room with the Blue, Red, and Purple chip challenges from the girls who sell you the chips and trade it with Trader during Mid-Day. (No idea if the girls will sell you anything when going into the Gamut early. If they don't, you may need to obtain the card and then start another playthrough to get to Mid-Day to trade.)
The Observer - I just can't stop looking into people. I do it 100 times...
Use Celeste's "Observe" skill 100 times. There's a counter for this displayed on your save file, so you shouldn't have an issue figuring out how much you have left. You can do all 100 observations in the one encounter. Get to the late game, find a weak enemy that won't kill you, set everyone to guard (except Celeste, obviously) and find something to hold down your spacebar until you get it.
Bit of a Smart Guy - OK, so you're bit of smart guy, hm? You like the numbers, especially the correct ones. You get genius skills for being a genius, and now all of your friends are like that too. Talk to Xal on the screen where the paper hero joins your party until they tell you the smart numbers for everyone in your party. (No clue if these are randomly generated or not). Raise any character's INT stat to the value given by Xal (I chose the Doctor, his was 21 for me) through the Gift Pixies. The achievement only popped for me when talking to the Gift Pixie outside the entrance to the Desert through the museum.
Cheese's Passcode - This guy really needs a better passcode.
This one has to be done in the morning. One of the loading screens at the save stations will have a 6 digit combination on it. (It's 440446 if you don't feel like searching for it.). Put the code in the keypad and enter Cheese's shop to get the achievement.
Nickel City Menace - What did I do???
This one has to be done in the morning. On the screen where you get the ranger, talk to one of the npcs just below him and tell the bored man to jump off the building. Talk to the guard blocking the entrance to the beach about the building jumpers. Go back to the man and fight the guard to get the Nickel City Key. This key lets you enter the Gamut early, and doing so nets you the achievement.
Gallery Assistant - A Gallery Assistant supports the daily operations of an art gallery by managing both administrative and hands-on duties. They welcome and guide visitors, handle artwork with proper care during installation and storage, maintain inventory records, process sales, coordinate with artists and collectors, assist with exhibition openings and events, and handle general office tasks like correspondence and database management. The role requires strong attention to detail, art handling experience, and excellent interpersonal skills to effectively interact with artists, collectors, and gallery visitors.
This one is tied to unlocking a secret party member. First, you must tell the rabbits in the 3 major areas to leave. The one in the garden is found in the large area where you fight Medici, the one in the paper zone is found in the lower left house, and two can be found in the mines just past the pirate but only the one close to the chair and tables needs to be sent away. You must also have 30 blessings. After, go to the family room and speak to the leather rabbit between the two "rest" signs to recruit the Gallery Assistant and get the achievement.
Paper Crane - It's just a Paper Crane, took so long to make.
Solve all 3 of Pixiv the Sphinx's riddles in one playthrough. The riddles appear at Morning, Mid-Day, and Evening. The answers are Happiness, Music, and Microphone.

The following achievements will either kill or softlock you, be sure to save before getting them!
Trolled - A powerful being from Above has decided to trick you. You are so helpless.
Enter the shortcut to Fabrizia's party area near the desert's large enemy craft. Wedge your way into the crowd until the Sun NPC spawns.
Heaven - This place is so lame. All I do is stand on clouds and spy on people all day. Well, and the hot tubs.
Talk to Rocky in the morning (possibly also during Mid-day and/or Evening) and agree to go to Heaven with him.
Redman - You just got Redmanned.
Tell Mr. X you HATE REDMAN!!!! and leave the house. Justice will be swift and painful.

Doing Nickel City Menace will let you get into the Gamut as early as you wish, which is really useful for getting these.

Dogstyle :D - Woof woof meow meow, moo moo cow cow, oink oink scratch scratch, chirp chirp, hatch hatch.
Set the paper king (located in the petting zoo, just left of where you find the ranger) on fire to get the dog tag. (I'm fairly sure the dog tag is needed to unlock these costumes. If it isn't, let me know!) Go to the costume woman and select "Bestial and Fluffy".
Retro Blasted - You're incredibly tiny now but it feels familiar. Yeah, this is an interesting lifestyle.
Go to the costume woman and select "Cute and Classic".

The Dark & Stormy costumes don't have an achievement just for wearing them like the others, but there are multiple achievements tied to them.

CopperOdysseyPachi - It's dark and stormy, I'm destroying the bees for a high score!!!!!!!
Enter Dark & Stormy mode in the Morning and go back to the first screen of the game. An enemy will have spawned, which is a bunch of bees. Defeat them to get this achievement.
Extreme Dark Awesome Legend - Destroy the 3 Large Enemy Crafts while being in Dark & Stormy Mode.
After starting the game in Dark & Stormy mode, beat the three large enemy crafts that advance the time to night while remaining in Dark & Stormy mode.
Worthy Sacrifice - I'm slowly dying but I really do need to draw right now. That makes the art better, right? Or so I was told.
Go to the area where Fabrizia lets you draw a picture while in Dark & Stormy mode and start drawing.

A HUGE thanks to Hainiryun and Ovi for helping out with Redman Saves the Day, Cheese's Passcode, Nickel City Menace, Paper Crane, Gallery Assistant, and Bit of a Smart Guy. Let me know if I can improve this guide at all!
Thank you for reading! And remember...

RESPECT THE CRAFT!