An operating system

I would usually recommend a patching method rather then a full download, I think a good place to start would be here: https://www.reddit.com/r/GenP/

If you already have Nextcloud running you can use the Nextcloud Forms app

Honestly, I would say because you just have less struggle. I had just a lot more problems when I was using Ubuntu instead of Debian. But I thinks it’s mostly personal preference

I don’t know about photoprism but I guess that’s not going to work because it’s really hard to provide features like face recognition and a web frontend with a zero trust setup. So if you just want the to have a automatic photo backup you could take a look at encrypted folders for nextcloud