I have the same problem, Nvidia RTX3070 with the newest driver installed. Now I tested a bit with the NVIDIA control panel settings.
If I only set the Monitor Technology to Fixed Refresh, the mouse lag was gone but I get a blackout of the hole window every few seconds when I change the tool for example.
Then I only changed the setting for the Preferred Refresh Rate to Highest available forced for the Affinity Applications, I don't changed Monitor Technology and had G-Sync still enabled. This fixed all the problems finally! 😀
This is really a strange behavior of the Affinity apps, never got such a problem with other 3D accelerated applications.
Cheers,
Sven