DSC is not a function of the cable. It is compression done by the GPU. With compression, an error causes greater artifacts in the display. Without compression, an error is less noticeable.
Errors increase when you chain cables together, especially if they are passive (don’t contain a retimer or redriver or repeater to boost the signal for the next part of the transmission). I think the KVM is passive. I wonder what kind of switch it uses – or what components exist between the inputs and output of the KVM?
Is the 3090 transmitting HBR3+DSC or HBR2+DSC? I know that the Apple Pro Display XDR will use HBR2+DSC for 6K 60Hz 10bpc (1286 MHz). HBR2 is only 5.4 Gbps. HBR3 is 8.1 Gbps so it will have more errors. A G9 supporting 5120×1440 240Hz uses a pixel clock of 2018.5 MHz which can be done with HBR3 at 8bpc 4:2:0 or HBR3+DSC (at least 8bpc, not sure about 10bpc; 4:2:0 is a simple 2:1 compression method – DSC can do up to 3:1 and is smarter about what information it throws away).
Shorter cables may help. Maybe a DisplayPort 1.4 repeater will help. Maybe you can use an MST Hub as a repeater.