I have seen discussions about a duplicati server/client relationship here before but this idea is (somewhat different).
Sure a centeralized server would be nice as it would be able to centrally manage configs and see backup status.
I think another possible benefit could be a built in transport protocol server (like sftp mabe) listening on a nonstandard sftp port. So this server would have storage capabilities.
Finally, it could possibly offload transport and CPU tasks. For instance, the server could handle backup validation rather than client downloading files and testing like that. It could also be used for rebuilding the database of the client because the client should be able to say “what do you have” and the server could say here is the file list and a hash or whatever. Limiting or eliminating, the need to copy everything back to a client over the internet to validate what is/isn’t backed up.
Problem is I have no idea how to make this. and it would require significant amount of time i am sure. but it may be worth it. i guess