[Dagonfly]

That seems about right.

> which is also optional, I believe

Compression support (DSC) is mandatory since DP 2.0. So running DisplayPort Alt Mode 2.0 (which is basically "DP 2.0 over USB-C") should guarantee DSC support. The new USB4 Version 2 also bumps tunneled Displayport to the latest DisplayPort 2.1 spec. Then you can tunnel up-to 80 GBit/s as well (without using alt-mode).

Thunderbolt 3 is a funny protocol, because in reality only Intel made PHYs for it, so the DP version is bound to whatever Intel controller you've got. The 6000 series controllers have DP 1.2, the 7000 series has DP 1.4 and DSC.