https://docs.armbian.com/User-Guide_FAQ/#why-does-hardware-feature-xy-work-in-old-kernel-but-not-in-more-recent-one
mixed up. 5.10.y is legacy, 6.1.y is vendor.
While it is great to have mainline for this board it is far from feature-complete. If you need more hw features to work you have to use vendor kernel branch.
Have you tried to lookup headers via apt? If no success, try building your own kernel packages, or an image with matching headers pre-installed, with the build framework: https://github.com/armbian/build
Sometimes it can happen that headers are not available for various reasons. Uncaught packaging failure, mirror unsync, ....
Still interesting though since IIRC driver for 8812au should be included by default. However it might have been disabled due to build errors or whatever...