I had a UDM Pro and wasted a ton of time trying to get it to work in the set-up you described and even tried Untangle in bridge mode. Ultimately I switched to Untangle and bought a used CK Gen 2+. I'm back to an incredibly stable network. The UDM Pro was constantly causing issues, chasing beta releases and spent more time in SSH than I had hoped. I'm using the most recent GA 5.x and avoiding 6 until more stable. In hindsight I'm not sure why I wasted so much time on the UDM Pro!
As a home user the only thing I'm missing is mDNS support which Untangle has known about since 2018 as the top requested home feature.
Untangle in Bridge mode impacted my upload speeds on FTTH PPPOE Clink.