Well, most things that messed up the game weren't caused by the choices I made in the jam, but rather by adding things to the game that would make level creation much easier later in development, which on the other side completely destroys the jam version. The only thing I changed was the main tileset, which used to consist of just 4 tiles that I would manually adjust to fit the level, but now there are 47 tiles, and I also automated the process.
Since most of you pointed out are objectively bad things, I will try to fix them. But personally I like how the track of the soul still exists if you are possessing something. I'm not sure what I will do about that. Maybe decrease the length of the track or make it so you can turn it off.
Everything else should be easy to fix. I don't know what's causing the lag during the first boss, but why fix it if it isn't even in the game. It will be similar, but since the final boss in the full game will be different, I need to remake it.