This is experimental, note that not everything will work, will work with the Linksys WRT series or devices with no WIFI, I had tested in my WRT 3200 and the espressobin.
If anyone wants to give a try this is my source tree for kernel 5.14 :
It was recently altered to remove the 37xx from being impacted, it came into existence due to an issue on wrtpac devices back in 4.x?? days I think; see commit, but as it says in commit, it is unclear if anyone has looked at it since.
Just took a stab at building a clone of your 5.14-rc4 branch and it fails for me. Wondering what flavour of GCC you are using, I tried both 10.x and 11.x resulting in same:
I use the default version 8 , Toolchain Options unchecked, before to change to GCC 8 delete /tmp folder or make dirclean.
I got the same error when tried GCC 11 binutils 3.7, changing to binutils 2.36.1 fixed.
so no all things WG, but that is pretty small; did not look to see if zx2c4 is doing anything upstream.
Flashed and booted on a mamba, but keeps generating a kernel fault, keeps running though. Have not tried on the rango I have here. To the note in OP re. wifi, I left mwlwifi in build, not sure if that plays a part here.
Took a clone of your latest 5.14-RC5 with WG fixup, built and flashed to a mamba. The kernel faults seen on RC4 are no longer, everything that I have tested appears to be working. The only anomaly seen on this target device is PWM (fan) is not working (was fine on RC4).
Edit: also the radios are up and running on target device:
I just added bonding to the RC5 build I had in place and uploaded to my drop if you want to test whether 5.14RCx resolves your issue. Images to be found from link on my avatar.
If you have not previously flashed an image to your mamba that enlarged the kernel reservation space you should use factory image and force. Whether sysupgrade or factory do no keep config.
The LuCI interface does not properly populate the list of devices, thus you have to select the blank entries in order to save LAGG interface and manually edit /etc/config/network file to add devices.
Once I manually edited the /etc/config/network file, reboot would hang, thus unable to test bonding.