Synology NAS with their Photos app would be a logical next step. It's not a drop-in replacement for Google Photos; it's more rudimentary, but you'll have your own private cloud where you can securely store and access your entire collection on any device.
Discussion
Thanks, this looks good! I was also hoping to find some docs alternative which they are offering. Now I'm reading more about their NAS models.
You can use it to host your own instance of NextCloud.