I've found this to be a good resource for discovering fun custom firmwares!

https://thefirmwarehub.com/

I’ve tried a few, I liked Jiant’s workflow the best so far (I also tried sloop and skiff)

I just wish it had proper 909 & 303 emulation

I've used sloop for a groove box firmware. Pretty nifty, like it more than the other groove box firmwares.