Yes, that’s the main reason to run the vendor kernel. Mainline support is improving all the time though.
I believe I needed the vendor kernel to use video through the USB-C port, and to use the HW acceleration for transcoding in Jellyfin. This situation may have changed since my last attempts.
I think https://gitlab.collabora.com/hardware-enablement/rockchip-35... is still the best reference for mainline support of the RK3588. As you say, DP alt mode and video encoding are totally unsupported right now. Hopefully things will keep progressing; it's a very feature-rich platform, and I think it will have some legs even after it is no longer the compute king (e.g., the RK3688 is on the horizon).
I believe I needed the vendor kernel to use video through the USB-C port, and to use the HW acceleration for transcoding in Jellyfin. This situation may have changed since my last attempts.