I’m building one[1]! Please sign up for the beta if you’re interested, I could really use some early feedback. :)

[1]: https://ente.io

For automatic backups of photos, this is built-in functionality of NextCloud. That's what I currently use.

My plan is to then identify a good photo organizer / viewing self-hosted application that does non-destructive organizing, since NextCloud's photo viewing is garbage.

It's amazing how there is no good Flickr in 2020

Is your focus area for mere hosting of photos (such as for availability/backup)? Or, are you more interested in being able to share tour photos? Is there a particular feature that is most important for you related to a G Photos alternative? I think if you post that, perhaps viable suggestions might be offered that would be best for you; instead of some general "try XYZ because it is randomly better!" sort of comment.

If you want to self-host, you're welcome to try out PhotoStructure: https://photostructure.com/faq/why-photostructure/