I'm not knowledgeable at all with your subject. However Google is my friend [somewhat]. Looking at the spec's of DP 1.4, it shows:
"DSC is a compression algorithm that reduces the size of the data stream by up to a 3:1 ratio.
[22] Although not mathematically lossless, DSC meets the
ISO 29170 standard for "visually lossless" compression in most images, which cannot be distinguished from uncompressed video.
[25][26] Using DSC with HBR3 transmission rates,
DisplayPort 1.4 can support 8K UHD (7680 × 4320) at 60 Hz or 4K UHD (3840 × 2160) at 120 Hz with 30 bit/px RGB color and HDR. 4K at 60 Hz 30 bit/px RGB/HDR can be achieved without the need for DSC. On displays which do not support DSC, the maximum limits are unchanged from DisplayPort 1.3 (4K 120 Hz, 5K 60 Hz, 8K 30 Hz).[27] " https://en.wikipedia.org/wiki/DisplayPort#1.4a However you would think it would support your monitors.
Looking at the GPU specs, it also states 4k 120hz, 8k 60hz, so it makes me wonder if that is the highest supported.
I am wondering if you lower the refresh to 120hz, if that would make any difference?
Hopefully one of our truly knowledgeable member will give you info that really helps. Wish I was more helpful.