it’s very easy to downgrade and even dual boot different versions. The only main thing preventing third party stuff from working on newer versions is rm2fb support, which is
what allows the apps to interact with the display, which is part of xochitl(note taking software by rM) and changes every update
most hacks I know are third party apps, and the one hack that adds a whole load of functionality to the main app is DDVK-hacks, which isn’t available yet. I use KOReader to read manga, and yaft with the Type Folio to run VIM. It all depends on what’s there(or what you can make yourself ;)) and your needs