Edited firewall config post above to make it easier to read. Also realized I forgot to recreate a WAN_TO_DMZ rule allowing all traffic into the DMZ zone (that's the whole point after all).
Firewall rules should be good but again the issue is when committing all of them, I get the weird error in OP and then the router becomes unmanagable. I've tried to find how to remove eth3 from swtich0 but the only thing I came up with was a post from 2014 and the commands don't seem to work: https://community.ubnt.com/t5/EdgeMAX/How-to-remove-an-interface-from-bridge/m-p/1024245#M42994
Any other suggestions on that would be helpful! Nevertheless, I will keep reading.