Because your unused characters get some AP after every battle, it's almost always beneficial to have a slightly larger roster than you might be used to with other FFT-alikes, as it will open up classes more quickly and give you better flexibility to deal with different battle conditions.
A large roster is also helpful with how Injuries work. If you have a character fall in battle, they will become Injured, which gives them a stat penalty until they heal, and they can't heal until they sit out a battle. On higher difficulties, Injuries can stack and even become permanent. It's never a bad idea to have three or four characters on the bench, ready to go, in case you have a handful of your team knocked out in a battle.
Keeping your levels on pace with the story missions is important, but the battles are a lot more reliant on you having a strong strategy and a good mix of classes that will deal with any stage's particular challenges. Don't discount buffs and debuffs, they don't last long but they can save your hide.
Poison effects show up early and stick around for a bit, and the continuous damage can be punishing. Healing it quickly, prioritizing taking out enemies that poison you, and equipping poison-resistant accessories will all help with some tough early-game battles.
The storyline characters in your roster are good, and should be used in Patrol battles if/when you're leveling up, as they'll be required characters in a lot of battles. That said, for various story-related reasons, you may not want to hinge your entire battle strategy around them or specific class builds for them.
Magic resistances and weaknesses can contribute to a huge difference in how much damage your spells do, it pays to keep an eye on them when choosing your roster for any given battle.
Ranged weapons and attacks do not use line of sight, so you can safely shoot "through" your own teammates and obstacles.
Even though you can and will have two active classes (like Mercenary/Mender), you will ONLY gain Ability Points (which unlock skills and passives for that class) for the primary class. Your Mercenary/Mender can use the Mender's healing spell as much as you want, and you won't gain any progress at all towards improving your proficiency with Mender.
If you want to actually gain Mender AP, you'll need to use the Class Change (which isn't as permanent as it sounds like) to switch your primary class to Mender. There is another way - you slowly gain AP in a class based on what your other active party members primary class is. The game calls that Vicarious AP. Your Mercenary/Mender won't gain any Mender AP itself, but if you have a Mender/whatever in the battle, your Mercenary/Mender will gain about half the AP the sum of all the Mender/whatevers are gaining.
This means that you'll probably want to frequently make use of Class Changes to prioritize what you want to be learning.

I cannot get over how the in-game sprites look like they're from Maplestory compared to the world/portraits

I got this and the DLC on sale for the equivalent of less than 5 USD.

This game brings out the nostalgia in you regarding older turn-based RPGs, which was clearly the developers' intention. I would say they delivered on the gameplay and that the class system and combat are what made me want to play this. The story, art, and other elements were a bit lackluster because the development focus was on the gameplay. I would still recommend this if you enjoy tactical gameplay because it's not as if the story and art is THAT bad