HDMI 2.1 is basically blocked by the HDMI forum:
"... This isn’t some unsolved mystery buried deep in kernel code. The issue sits with the HDMI Forum, which controls the HDMI 2.1 specification. To implement it fully, you need to agree to licensing terms that clash hard with open-source licenses like GPL. ..."
Source:
https://www.makeuseof.com/hdmi-forum-is-holding-back-linux-a...So I can perfectly imagine a small hardware vendor like Framework being unable to get support for this.
Perhaps DP is a better solution for your use-case?