Problem is that Seedvault depends on the same functionality as "adb backup" which Google has deprecated and said they will remove it in the future. That's why a new solution is needed. As far as Seedvault is concerned, I have tested restoring individual apps from APK while a USB drive with a Seedvault backup was plugged in, and the data was restored correctly. Whether that is enough to restore an entire phone is a different question. There are a number of apps that successfully opt-out of being backed up.
A future solution should try to make use of mechanisms proposed by Google instead of working against them. That's why using BackupAgent seems to be a good idea. A future solution should also support more than one storage backend, that's why integrating rclone seems to be a promising option; one integration point, multiple backends.