>as far as I understand the direct solver cannot be accelerated with a GPU.
As I said, it can be accelerated with GPU, unless you use anisotropic media in your design. there may be more restrictions, which I did not find. ANSYS treacherously refuses to disclose all the limitations and requirements. But with simple copper on isotropic PCB the nVidia Quadro RTX board gives a well visible acceleration at the matrix solving step.
I was running it with Quadro used for the main display at the same time.

The restrictions on use of Geforce are purely artificial, probably defined by the management team.