I re-packaged the bin files you sent over and re-built the OpenWrt images. Both SXR and SXS loaded up fine so I must have just had older files.
I notice with the old and fixed bin files I can't select channels 12 & 13 for the 2.4GHz radio however that looks to be a firmware issue https://forum.openwrt.org/t/adding-openwrt-support-for-xiaomi-ax3600-part-1/55049/2487 rather than specific to the board file.
Otherwise, I've had the SXR & SXS running as a mesh using one of the 5GHz radios as backhaul for just under a week without any issues.
Will report back if any issues. I was unsure about the build recipe for sysupgrade but it is working, so I will revisit that when I have some time and will try and document a possible install process.