A data point for developers working on the MT76 (kmod-mt76x0e):
I compiled a git trunk (20191117) sysupgrade image for Asus RT-AC51U, and configured it to bridge 5GHz and WAN. I don't use 2.4GHz, but I assume that works too. The router itself is only accessible via the LAN ports. (I really needed only a wireless bridge to WAN, no NAT or anything like that.)
Everything works fine, including 5GHz transmit power. I get 50% signal through three interior walls at around 8m distance, around 100% signal at 8m line-of-sight.
(Oh, except for the 5GHz LED; OpenWRT uses the LED marked for 2.4GHz for all WiFi activity. I don't mind; definitely not enough to investigate where that LED is connected to, so I could add it to the DTS. I did add the 5GHz-6GHz frequency range to the DTS, though; I don't know if that matters any.)
As to configuration, I literally bridged the WAN and WiFi; no relayd/trelay etc. needed.
Of course, I had to remove the default firewall rules, as those would block DHCP et cetera. As it is, I have no firewall rules set for now, but might set them later. (I do run fail2ban on my machines, and if I encounter a pattern I don't like, I'll drop such packets at the firewall.)
The LAN is not bridged (as it is in the default configuration), the LAN ethernet switch ports are their own standalone interface. It does provide a 192.168.240.x DHCP addresses for easier access to router configuration (again, only possible via an Ethernet cable to one of the LAN ports), also mapping "config.lan" to the LuCI/DropBear IP address (192.168.240.1). The WAN-WiFi bridge is unmanaged (no IP address).
In this configuration, wireless clients will get their IP address from WAN DHCP servers (upstream; in my case, from my Internet Service Provider).