Microsoft’s Wolverine for the TCP stack was not available until Windows 3.11. An argument could be made that these systems are defacto air-gapped as they cannot communicate with modern networking.

J1900 has no hardware switch. Every packet goes through CPU, so even LAN to LAN uses processing power. Add pfsense to the mix and it’s probably choking.

Not 6E but the GLINet Flint 2 runs a flavour of OpenWrt 23.05.

I own the original Flint and I have been really happy with it. It's has a "fisher price" mode for most people; but then you can also log into Luci and really tinker under the hood.