Oh no, I don’t think it would be able to pull from iCloud directly, I believe there is an option to delete uploaded photos on the iOS device after uploading in the Nextcloud app but I don’t know if that is automatic or requires the user to acknowledge a prompt unfortunately.
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.
bad news, Proton’s official reddit account doubled-down on it as a company position (edit: but it was also him too)
The Trust Thermocline!
It’s a very real belief, lot of folks here weren’t around to know the “before times” and nothing is ever real until it happens to them.