So the app I am currently writing just went into open beta. It is a Nextcloud first gallery app for Android. If you are interested check it out: https://play.google.com/store/apps/details?id=com.github.vauvenal5.yaga
And here are the docs: https://vauvenal5.github.io/yaga/
Keep in mind though that it is in early beta state. So it is fairly stable but missing a lot of features. Also I have tested it only on a very limited amount of devices.