OpenWrt runs on the RPis. Why not flash it & give it a shot? It sounds like you’re going to have to do some custom firewall chains so you’d probably end up using LuCI for an advance firewall config rather than the convivence of GL’s GUI.
Note the Burme 2 is currently at GL firmware 4.2.1. That runs OpenWrt 21.02. This tidbit is important as OpenWrt is about to change over from iptables to nftables for the firewall in the upcoming v. 23.x.
You can always transfer over those confs to the Burme 2 when you’re ready to pick it up: