Good to hear you got it working again.
The CRS306-1G-4S+IN isn't really meant to be used as a router, it is a switch with some router capabilities, and those router capabilities are there primarily for you to manage the switch. You would be much better off using the router in the UDM for inter-vlan routing, and then keep your top talkers on the same vlan of the switch, so routing isn't needed. Or buy a RB5009 as your router (that has some switch capabilities).
See the CRS306-1G-4S+IN performance
Test Results, and compare the switching packets per second to the routing packets per second. The CPU in the CRS306-1G-4S+IN is 800Mhz with 1 core. That's just over than the original Raspberry Pi from 2012 (that had a single core 700Mhz CPU and 256MB of RAM). The original Pi Zero from 2015 had the same processor BCM2835 as the original Pi, but a newer gen, and it was clocked at 1Ghz and 512MB of RAM.
I have no UniFi routers, but you can find youtube videos on how to setup vlans on the UDM. But the link between the UDM and he CRS306-1G-4S+IN is going to be a bottleneck for any inter-vlan traffic.
What exactly did you mean by "I am trying the routerOs rather than the switchOs so I can hopefully get the SMB portion working to." in the OP? If you are planning to do that with the CRS306-1G-4S+IN, I think you will be disappointed.