vmbr0 without physical uplink

Hmm, yeah, it sounds like it has created a bridging loop, which amounts to a broadcast storm, and that is the behavior you saw with it shutting down your network, it sounds like. It may not work since you are combining the two bridges for the one VM if I am thinking about this correctly and that is what causes a bridge loop if there are more than one layer 2 path between devices. So, it may not work to try to include a virtual interface from the Linux bridge.

I guess next time I feel like breaking things again I should try it again and see if I'm able to access my other services and if it's just the Proxmox webGUI that I can't reach. Because if I created a loop, that means the bridge was doing something and unplugging the physical cable should break the loop. 

@life-from-scratch it would be interesting to see what happens with more testing and see if you are still unable to get to the web UI on the same setup or if this was due to something else at the time. Keep me posted on what you find there 👍 

Well.... I unplugged the cable from my switch back to the Proxmox node and inside PfSense just bridged the LAN and virtual interface together and for some reason this time it just seems to work. All my services seem to be up and I can access the Proxmox dashboard. So I'm not sure why it didn't work when I initially set it up, but it does now.

This opens up a lot more options for machines to run a "forbidden router" setup on like Hardware Haven did. I get why bare metal routers are preferred, but I also like the idea of being able to run other critical services (Home Assistant for me) on the same machine. Running like this it should work with only two nics. One for WAN, one for LAN and not needing a third to for the hypervisor. 

@life-from-scratch that is great! That is cool that it works as you expected it to the first time. Keep me posted on how it performs and operates moving forward.