For those who want to keep using their Apple devices, I highly recommend Nextcloud as an iCloud Photos alternative. I've been running an instance of it on a little Pine64 for quite some time now without any issues.
Though I'll still the say the crux of the issue is that the majority of users _won't_ have iCloud Photos disabled, and thus have their privacy violated.
How large is your photo library / how would you compare NextCloud photos to Apple Photos? I'm starting with testing Synology Photos which has at least basic local people tagging (a must with small kids). Feels like a big tradeoff with all the self-hosted options. I've heard NextCloud is slow. PhotoPrism looks awesome but no mobile client (yet) which makes family adoption tricky.
I'm using mine as a general cloud storage alternative as well, so it's using a little over 1TB. About a quarter of that is photos. So not too huge, considering they're saving at full resolution.
Though I'll still the say the crux of the issue is that the majority of users _won't_ have iCloud Photos disabled, and thus have their privacy violated.