As somebody who has tried and tried and tried to do exactly what you want to do, albeit with a Creta and a Slate Plus, I will say that it is theoretically possible, but with serious caveats.
The GLi firmware, with very limited exceptions such as Brume 1, does not support USB wifi dongles. It will break in various ways if you add one, often simply because it doesn’t expect an extra radio device to be present, or because the dongle is now showing up as
radio0 instead of the the radio GLi’s firmware expects to be there, etc.
If you do successfully add a dongle and get it working for uplink, which can in some cases be done thru Luci or the command line, various parts of the GLi firmware will become fragile, meaning that once you’ve got the setup working, if you attempt to subsequently change settings in GLi’s firmware, it may break. And by break, I mean up to and including breaking your entire setup, not just the setting you are changing.
You can of course attempt to do a setup like this with vanilla OpenWRT if you get a GLi device that’s supported by mainline OpenWRT, I don’t know if this is the case with Beryl AX.
In either case - GLi firmware or OpenWRT - you then run up against the problem that USB wifi dongles are not well supported in Linux, often times the drivers are poor quality, etc. This is improving with each release, but it’s a crapshoot trying to figure out which hardware is truly well-supported and which isn’t, unless you want to use certain ancient 802.11n dongles.
You may then find a well-known list of USB wifi dongles which “work well on Linux,” but if you purchase one of the newer ones, you may then subsequently find out that bugfixes & patches which have made it into new Linux kernels used on desktop systems, may well not have made it into OpenWRT at all, let alone the older kernels still in use in some GLi devices.
I’m not saying you shouldn’t try doing this. I have a bunch of dongles and hardware bits and give this a good college try every so often, because being able to use good directional antennas (on something other than a Shadow) would be a huge win for me. But so far I haven’t had a great experience, and I’ve been taking a break from it for a while to let more
mt76 bugfixes land in OpenWRT.