1. Install Nextcloud Server.
2. Install the Nextcloud Client on your phone.
3. Set up autosync of media files (photos, videos) with local deletion enabled.

Bonus Tip: Don't expose your Nextcloud server over the public Internet, hide it behind a Pfsense firewall and keep a persistent VPN connection open.

This way you never keep personal or private data on your phone. The moment you take a photo, it's immediately synchronized to your server and deleted afterwards.