Mac Mini drives Samsung UltraWide 5k with 5700XT eGPU
Although Apple talk 60Hz - and these monitors don't like 60Hz - and Samsung assume we all understand HDMI tops out at 4k, the best options are DP cables and T3 to DP cables. (StarTech do them)
Running UltraWide at lower resolutions defeats the reason for having them so make sure you focus on DisplayPort cable outputs to avoid HDMI bottlenecks...maybe HDMI can go 5k ... I don't trust HDMI more than 4k.
So there you go, Big Sur drives a 5k 1440p display at 120Hz. 🥳
Thats a monster of a monitor, but as far as I understand thats not 5K (could be wrong), the one you linked to is 3840 X 1080 which is 1080 wide screen. 4K is 3840 X 2160 and 5K is 5120 × 2880.
Regardless, I'm sure it looks great! I can't seem to get my regular 4K monitor above 60HZ in macOS (it does 144HZ), how did you do it?
2017 13" MacBook Pro Touch Bar
GTX1060 + AKiTiO Thunder3 + Win10
GTX1070 + Sonnet Breakaway Box + Win10
GTX1070 + Razer Core V1 + Win10
Vega 56 + Razer Core V1 + macOS + Win10
Vega 56 + Mantiz Venus + macOS + W10
You can't trust what the macOS GUI is showing. Use the AGDCDiagnose command to check the connection type (HBR3 or HBR2? 4 lanes?), the output resolution, and the output pixel format. It will also show if DSC is supported and enabled.