When I’ve searched for OpenWRT/LEDE bridging configurations, they usually tell you to just set up a DMZ, and turn off all the other stuff (DHCP/Radio/etc)
I’m not sure how much of a bottleneck would still exist after doing that.
You’re gonna want to use QMI to get LTE (wdm0). If the interface says “3g-modem”, it’s in USB mode, and in my experience, you’re not getting more than 20Mbps up or down with that configuration. My speeds jumped to the mid 40s down and low 30s up after making sure it was in QMI mode.