Jozek
I have a set of GPU drivers that you might want to test. Indeed sadly Debian support for this board/GPU does not yet exist, even on Buster, only Mali T62X and T76X are available.
I will pack and upload the driver packages later and share the link.
We reopened the related GitHub issue: https://github.com/MichaIng/DietPi/issues/1794
Checking/testing drivers/libs provided by Ayufan currently, but they come with some dependency issues since made for Ubuntu. RockChip sadly provides only armhf drivers for this chip but not arm64 ones. At the end of the issue I posted links to another (older) package archive provided by the ARMbian community. This is only made for their Ubuntu desktop image but AFAIK the packages do not come with any hardcoded dependencies, so can be installed on our image as well, after installing Xserver (or a desktop) via dietpi-software to have the base graphics/OpenGL libraries installed.