Hi,
First make sure that your VM is using the latest virtual hardware.
With the VM shut down, not suspended, go to menu "Virtual Machine" -> "Settings" -> "Compatibility" and set it to the latest available.
If that was already the case then have a look at the following thread for a few possible workarounds:
Fusion 12 w/ Big Sur Display Flicker
You might already have seen the thread (not sure, but your post suggested that you did)
If that still does not help then make sure the VM display has enough RAM set for the display under menu "Virtual Machine" -> "Settings" -> "Display" -> "Shared graphics memory".
hope this helps,
--
Wil
