Google photos is canceling free unlimited photo storage. Many need an affordable solution to store a few hundred gb family photos synced automatically from the phones of the family.
What about extend the functionality of the companion app to sync picture data to the storage managed by the family’s always on center?
Is it too far from the original goal?
HI is not designed for that !! Just use Nextcloud (Free, open source solution where you are not the product 
that has that feature integrated and basically allows you to get all Google “features” in your own private cloud ! I offer that solution at quite a number of customers that are very happy with it 
Another solution would be the app SMBSync2 in addition with the SMB Share addon. It does not have the funcionality of google but it backups whatever you want and has many features.
I’ll try
It works, but too slow on an rpi4 to enjoy.
What storage do you use in your rpi4 ?
I just had a try with the 64gb SD card the system is on. But adding a few 10 photos was already pretty slow.