I might have a misconception of what back up is.
Right before writing this, I used an ipad to take a picture of one of my cats. That ipad has the filen, nextcloud, and sync apps. I added the same photo to a test folder on each of the services I just mentioned. I can see the picture in the test folder of each service on my desktop.
Those are all free accounts, yes. I am not a paying customer yet. If that is not a backup, I don’t know what to call it.
The pictures I currently have and want to save are on an external hard drive and on proton drive. Any (future) pictures I take are:
Because proton does not have a Linux app, the problem with this workflow is that I need to download every picture I want to edit, and then upload it back to proton.
The options I have are:
switch to windows (that is never going to happen).
switch to mac (I am temped, but the idea of having to buy a new computer every two years because they become obsolete bothers me a little).
stay on Linux and use nextcloud on an iphone, an ipad, a dell optiplex 7010 and a toshoba satellite (both with 4Gb RAM).
That was just a picture of the RPi4 I want to use for this project. Back in 2020 I ran gphoto2 on it and used it as an intervalometer.
Your suggested setup would allow two hard drives to be synchronized, but the web archive (proton) would still need constant downloads and uploads.
Free filen and nextcloud accounts have allowed me to do what I want… take a picture with the phone, upload it, edit it on that service’s folder, see the changes on every other device.