I haven't used PhotoPrism, but wanted to toss it out there as an alternative.
This builds the thumbnails in the background. I think it used to get triggered on upload but says it needs a cronjob now. Works OK on a folder of 40GB of photos.
What I still find irritating about photos in nextcloud is I keep mine in high resolution but nextcloud won't downscale them to fit the viewing device screen, so I use 20mb a photo with all the UX impairment one might imagine when actually a few kB would do. It'd be nice if it cached light, fullscreen versions and offered a "view in full resolution" button.
Yes, I have no idea what they don't do this. This makes the gallery barely usable.
More info: https://docs.photoprism.org/user-guide/settings/advanced/#im...