Which features are most important to you? Search/discovery, categorization, tagging, sharing…?
These days I usually just search the web for images and save them to folders on my computer. I have the folders synced to my cloud storage, so I can access them from any computer if I want to.
I think participating in communities which are centered around discussion of open source software is the ideal solution. That could be on Lemmy, Mastodon, GitHub, Reddit, Discord, wherever devs and users congregate (and, whatever platforms you find tolerable). I think the information you are seeking is too varied and in some cases subjective to be captured and parsed by an automated tool. And it would be great if you could help others by posting in those communities about changes that you are unhappy with, so others can make informed decisions.
There’s a lot to address here as you’re talking about hardware and possibly multiple levels of software.
Yes, you can do this with raspberry pi or any SBC or mini PC. Even an old desktop PC if space isn’t an issue.
In terms of photo management software, I really like Photoprism. Immich seems to be popular as well.
In order to get your photos synced to multiple computers over the internet (a good idea for resilience), you could look at syncthing. Alternatively, you could have one central server and one or two backups in different locations using borg backup or similar. In my experience, backups are easier to manage and make it easier to recover from data loss than replicating the current state of your data in multiple places. You can do both, though.
It’s a very worthwhile project, but may be pretty difficult unless you are already comfortable with server technology or are enthusiastic about learning.