Syncthing. It’s not so much a backup as redundancy, though.

I have machines in the network that rarely get powered on, however, so I could possibly consider them offline backups.

No email provider will go to court for you for €3.99 per month.

From the start of the article:

Key Takeaways

• Proton Mail shared payment data with FBI through Swiss authorities via legal treaty
• Credit card payments eliminate anonymity despite encrypted email content remaining secure
• Third known disclosure reveals pattern of Swiss legal compliance over privacy promises

Viewer? Probably just Quick Look. It’s built into the Files app.

If you want to edit, just use Apple Photos. Everything is in there anyway.