@Stach There plenty of solutions depending of the problem you have :
- You can backup your data on you PC with tools like syncthing.
- You can also self-host a backup solution on a private server and use tools made for this.

for the second solution, you can use :
- Etesync for your contacts/calendar/tasks
- Immich (https://immich.app) for your photos/videos.

If you juste want to backup your phone with data/apps. It depends of the ROM you're using. I use LineageOS and Seedvault for the backup.

And for your documents/other files, You can use a cloud storage respecting your privacy like proton drive.