Yep, you can do that. There might be just missing functionality (likely leds and anything controlled by gpio) but it "works". A lot of these mt7621 routers just have the ethernet switch on rgmii1 so it's pretty straightforward to do a device tree. We already have a fully working router (save for LTE) by the time this was posted (it didnt really take long since we have someone with experience on mt76 devices).
We've already got serial to the modem covered a long time ago

We're currently reversing binaries and documenting AT commands to not do a haphazard implementation of the LTE module. We also need to reverse a few usb control messages as not everything is done thru serial.
We're just doing this in our spare time and have other responsibilities so it's taking a bit to finish dealing with the LTE module.