I wonder if you could do similar to @rdlvm did here with sewing needles and to get access to UART without having to open the case, especially since the AX6000 seems to have ventilation holes straight above the UART pins;
You certainly can. Second time around I did it with a few needles as mentioned.
I would say the effort was about the same for me though, as it's just 4 screws and a plastic plying tool to get more direct access
If you haven't rebooted yet, make sure that the boot environment is ok (fw_printenv) and that mtd7 is the ubi partition (cat /proc/mtd) and if so, just a sysupgrade -n /tmp/squashfs-sysupgrade.bin with an stock layout image should fix it.
If you have rebooted already, it's going to be trickier...
If you have a custom build without kmod-mtd-rw and you still have the build, just add it in .config with "m" and run make and it'll build it as a separate module. It should be quite quick and you can load it in your current version.
Of course, if you've done any updates in the build enviroment, it won't work.
As far as I know, there will be no official stable release of ath11k (Wifi 6 = 802.11ax) untill the OpenWrt version 23, planned for autumn 2023(?). From my understanding, there won't be a single Wifi6 device supported in stable releases in OpenWrt 22.
Stable release = non-snapshot (not night build). It has nothing to do with stability of OpenWrt. It is indeed stable on my router as well (apart from some previous issues with specific client devices or ISP settings).