If we can find a way to block the DHCP responses from the plotters so that the only DHCP response a user device gets when connecting to the balance ‘Garmin’ network is from the balance, then that user device will be able to run the helm app and still be able to use airplay (with Bonjour forwarding turned on between the Garmin VLAN and untagged network).
A cheap Netgear 8 port switch between the garmin hub and the balance LAN would be able to block DHCP responses…