There exists a frontend called Proxitok, but I can't comment on how well it works or what it can do. The LibRedirect addon for Firefox can redirect you to the instance of your choosing.
Great addon in general, do check it out if you don't know it already.
I recently discovered Image Toolbox, it has the best editing capabilities of any FOSS Android app I've seen so far. That being said, if developing RAWs is something you need as well, I don't think there exists anything besides Google's Snapseed and Lightroom mobile.
As for taking pictures, I quite enjoy OpenCamera, but there's not much else than that.