tried this myself and found my warning confirmed with SV builtin webdav: it doesn’t work with nginx+php-fpm hosted Nextclouds atm, see the 0 bytes column:
despite that, this is the config
the path is as suggested in the mask /remote.php/webdav or if it needs to be a subfolder it’s like in this doc, for example if you create a ‘backup’ folder beforehand you can do:
https://murena.io/remote.php/dav/files/username@murena.io/backup/
The .SeedVaultAndroidBackup subfolder will only be visible when “show hidden files” in Nextclouds config in the lower left is enabled.
So my comment holds: device-local folder as destination (sdcard?) and a secondary process to transfer from there. Round-Sync is both an option to use directly through SV or as external transfer tool. But I found RS hard to setup and combined with SV setup (12 word passphrase) - it all has a lot of friction.