I was using 2 27" LG 27GM950-Bs (IIRC), that could do up to 165Hz and VRR on a 2019 cheesegrater Mac Pro, wasn't the cables, or the monitors, or the card.

People at the time were trying to figure out the math of "How did Apple manage to make 6K HDR work over that bandwidth?" and the answer was simply "by completely fucking the DP 1.4 DSC spec" (it was broken in Big Sur, which was released at the same time). The ProDisplay XDR worked great (for added irony, I ended up with one about a year later), but at the cost of Apple saying "we don't care how much money you've spent on your display hardware if you didn't spend it with us" (which tracks perfectly with, I think, Craig Federighi spending so much time and effort shooting down iMessage on Android and RCS for a long time saying, quote, "It would remove obstacles towards iPhone families being able to give their kids Android phones").