Thanks for this question, OP! I made the mistake of asking this question on Tachiyomi’s discord and they were very unpleasant about it (and apparently they don’t want people using all the same extensions too).
Some have already been mentioned here but currently I have been using:
MangaDex (I still don’t know how to make NSFW titles appear tho)
I’d recommend having your library spread through various sources as sometimes extensions stop working and you might need to migrate all mangas one by one
I was expecting they wouldn’t change the visuals and feel of the game so much, I guess. Don’t know if it was the idea but I tried for a few hours and ended up refunding it because I was expecting the same “feel” I had from the original/hd version and everything felt so… different. Maybe I need to give it another try but I remember at the time I decide to stick with the HD Version because it was what felt more familiar for me.