Immich is rapidly growing into the closest solution for replacing google photos. It’s not fully there yet, but there is very active development going on and it’s advancing quickly.

I've got a Synology NAS DS416play for several years now as a trusty backup/storage. But not using their Photo Galary feature. What I like about Synology is their long term support. They still offer software updates for this rather old model.

I'm still using Google Photos as main photo dumping ground as it has clients for iOS and macOS which automatically uploads all photos there and of course for the search and sharing feature. I'm even paying for the extra storage needed to host all the family pictures on a 2TB plan.

My concern having only the Synology would be that in case of fire/flood/zombies I'd loose all this stuff. Sure it's a 4 disk raid, but I'd have to make regular backups off-site. Which is pita and expensive as well. Synology does offer Backup to various cloud locations though. But overall I considered this more expensive than just paying for the Google Account.

Adding a factor to the mix, what about mobile clients?

Personally I use Nextcloud for the auto-upload, but it's more of a Google Drive/Office 365 replacement than a Google Photos one.