OP have you made any progress on this at all?? I'd like a setup pretty much exactly like you want. I've got my unRAID server and a couple PIs that I'd like to cluster together somehow either via docker swarm or kubernetes or whatever and I'd like to run nextcloud, vaultwarden, freshrss, swag and a couple other containers and have it so I can still use them if my unRAID server has to go down.