Syncthing synchronises these backups to my home server automatically, as well as the phone's user data (photos, mainly).
The photos are additionally sent to Photoprism.
I avoid Google's cloud as much as I can.
I used Nextcloud sync in the past, but found it unreliable.
What was wrong about nextcloud sync? I'm just about to set it up for myself.
With Syncthing, I sync to a directory that my Nextcloud user can access (a read-only mount), so I can still easily share photos using Nextcloud, for example.
(although it's unfortunate that the Android syncthing app is being retired. h/t for the heads up and the recommended alternatives)