In January I already couldn't get two seats next to each other for any of the remaining season games for me and my friend

Lots of tickets used to be available even shortly before matches up until last season or so when many bandwagoners came in.
Single tickets are still available, I recommend game against Stuttgart. Last time I went they brought pyro lmao and they fight against relegation so the game will have a meaning. Nürnberg for the same reasons, both teams have loud fans. Augsburg is a boring fixture and Hertha is a boring club but the game should be valuable because it's fight for EL spots
List of dates and games:
https://tickets.eintracht.de/
click "Jetzt bestellen" and then on the stadium layout to see available seats. Buy as far to the left as you can cos it's the home curva
How to buy tickets english guide
https://www.eintracht.de/en/tickets/
It's much easier than Juve tickets. You register, pick the seats, pay and print at home