How to get Ending 1

You can play the game normally, grinding Tokimeki with any character, as long as you don't get enough Tokimeki at the end for 1 Specific Character. Perhaps Limit it to Tokimeki Reached of 5 or less (I keep tabs of the number of times I got a Tokimeki for all them using notepad). That way you won't go through a specific route at the end.

How to get Ending 2

I've tried focusing on 2 Characters, grinding Tokimeki equally, I've reached 10 Tokimeki Count on both before 26th day ends. The Specific Route at the end randomizes but you can just skip that and then you'll see a different Ending.

How to get Ending 3

It's not that hard if you use your Save States very efficiently, You must watch / read each Character's Specific Route. For this I used 13 separate Save States so I can grind them all with ease (Of course you must still raise their TOKIMEKI separately on their save state until the end of 26th day) For Example: I've grinded Mia's Route and reached 28 Encounters with her, not all of them have events that give TOKIMEKI. After you finish with the last Specific Character Route, It will automatically proceed to the Ending.

To unlock all Bonds and Events on Extras for each Character

As you grind Tokimeki of each character, you unlock them one by one eventually but some of them you don't unlock even after you finished day 26 (Of Course your grind should be all Tokimeki Increases for that specific character. But there is a way, you will need to progress through the Specific Character Route to grind more TOKIMEKI, then you can just view them afterwards.