Yes, I've been using SFML up until now.
I already moved a small part of the project onto Irrlicht. I don't reallylike big bloated libs and Irrlicht provides everything I need. Besides, some parts of code will be reused so it's not that bad.
As for extensibility - yes, that is planned but not to that extent as you described. You already can tweak enemy ai to some extent with FSM. Plugins system is... out ofscope as of now. It's not out of question though.
I might create a patreon at some point, however this is not my main job. Just a hobby, so I can't really provide big updates every month.