This isn’t necessarily a 1:1 answer for you but it’s just what I have experience with: I used Nextcloud for a period of time and set up the app to upload which worked like any basic cloud storage service but what I’ve done since (I downsized) is just use the PhotoSync app and it’s companion on my PC. You can target a PC (like I do), WebDAV, FTP, SMB, etc. which may help with scalability.
This isn’t necessarily a 1:1 answer for you but it’s just what I have experience with: I used Nextcloud for a period of time and set up the app to upload which worked like any basic cloud storage service but what I’ve done since (I downsized) is just use the PhotoSync app and it’s companion on my PC. You can target a PC (like I do), WebDAV, FTP, SMB, etc. which may help with scalability.