I really like the filter selection. It's the only open source app I know of that lets me blur images and that's pretty much all photo editing I do on my phone.
FFShare works nicely for reducing file sizes, but I wish it was more forgiving and did the conversion in the background. More than once I waited through the conversion and then accidentally closed it.
Termux also works well. Some tips:
change the shell to something like Fish. Autocomplete helps a lot when typing on mobile
I think the archwiki is a good source for FOSS apps, but I'd certainly recommend GIMP and Krita for images. Davinci resolve is oss if I'm not mistaken(correct me if I'm wrong) and there's also Kdenlive.