A side note; am a longtime user of Pikture gallery app on android. Have the same issues as OP wrt Nextcloud and photohandling.
Did a quick search just now, and stumbled over some new info at https://www.piktures.app/.
"CLOUD ACCESS
Piktures let you access your photos on Dropbox, Google Drive and Microsoft Onedrive. Nextcloud is coming soon".
Dunno' how soon, "soon" is, but might be worth waiting for, as Piktures is IMHO pretty good in and of itself.
I'm migrating from Google Drive and Google Photos to NextCloud and also have been looking for a photo gallery app that can edit without having to download the file. I haven't found it. The closest I got is the Piktures app that reportedly will support NextCloud soon.