I ran an old thread where’s some adb cli introspection for seedvault, where you could enable it and set the default transport - [HOWTO] use Seedvault early to understand Android backup
That said, Seedvault got better-ish with A13/A14 and more warts removed with A15.
Seedvault in /e/OS based on Android 12 has a very old version.
Nextcloud as SV backup target (if not behind an apache) is unlikely to work, use the sdcard or internal disk and adb pull from there.