UPDATE:

upgraded my graphics card [to RTX 3060], which I was going to do before too long anyway; that in itself did not fix the issue - but it did go away, ie stopped happening, when I made the following changes in the registry:

Computer\HKEY_CURRENT_USER\SOFTWARE\Xara\XtremePro Perpetual x64\20\Options\Screen
IsRenderingRAMInfinte set to 0 [default is 1]
MaxRenderingRAM set to 3145728 [default is 786432]- this may not be necessary in practice...
SmartRenderingRAM set to 0 [default is 1]

after these changes the program ceased to crash without warning on zooming right in - sometimes got message about bitmap being too large to render, no surprise at this level as the artbrush texture is a bitmap, the program however was stable

it does mean that zoom screen redraw becomes visible on high render drawings, but that is acceptable if the alternative is to risk the program crashing out

I am still not sure of the cause of this, but the fact that it does not happen in my pro 15.1, but does in the later programs, might give someone in xara a clue