I am also not super happy with it, but: HDMI indeed regularly broke on this board, so I can understand that for this old board with low support state keeping things functional with a frozen kernel is pragmatic. This was not done or approved by the official Armbian Odroid C1 board maintainer, though.
It should not break DKMS, as the headers are kept omin sync. The headers package was upgraded as well, right?
That said, we could unfreeze the kernel in our fork. But would be good if someone could test HDMI then.
I am currently using this SBC as headless server, I was not even aware that the HDMI support was there at all (it never worked for me in the past since the adoption of a mainline kernel, I stopped bothering about it long time ago).
In any case yes, I upgraded the headers file as well.
The dkms module that fails to compile with the latest kernel package is: https://github.com/lwfinger/rtw88, while it compiles fine with the previous one.