My setup is a bit different, but maybe you can reuse part of it. Instead of using swarm for HA, I’m using proxmox. The LXCs have a failover to other machines, if they go down, and a static IP address, so if the http proxy machine goes down, it boots back up in another machine, with the same IP (and thus a working port forward).

This does mean that I have to keep the configuration in sync between different machines so my RPO is never too big, but for something like NPM where the config rarely changes, this isn’t much of an issue.