Currently my OBS instance in Windows cannot output 2160p60 to my Ultrastudio 4K Mini. Pro Tools can send 2160p60 video to it just fine, so I know it's not a hardware or driver issue, something seems to be wrong with the OBS implementation itself and the OBS forum people say to reach out to Blackmagic.
UPDATE: Going back to OBS version 28.1.2 makes the 4K output work just fine. It seems that OBS 29 broke the Decklink support, at least somewhat. 1080p still works in v29. 2160p, not so much.
Going back to 28.1.2 did not help in my case (at least not all the way). 29 and above stays black. With 28.1.2 I get a frame every once in a while. What driver version did you use? Since Mimolive also does not work I am not sure if this might not be a driver issue after all.
I do have the same issue. No output above 2160p25. Right to the point where dual channel SDI is required. Unfortunately I cannot test on 28.1.2 since it does not work well on silicon Macs.