I now repmoved and reinstalled the Quadro drivers, will see how that works.
The permanent disabling of Wayland and the advice to edit
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash intel_iommu=igfx_off" Seemed to have helped - the flickering seems to have stayed at the edges this time, not move in with every wake-up - now I will see if reinstalling the Nvidia drivers ([How to] Reinstall NVIDIA Graphics Driver) will help also.
Update: Seems this is as good as it's going to get, for now (this doesn't happen in Windoze, could it be something to do with DirectX?), but it's much better than it used to be. Thank you for the support.