Hi Gunter,

re:
> Once, that spiral is sorted, how would I go about creating the variable depth of the relief?

So one possibility for the variable depth of the relief could come from doing a sweep
with profile curves that have a peak and dropoff shape to them.

Something like this, the profiles point upwards in Z and it would be good to make a profile station
for every spiral turn that get smaller so that the sweep won't bunch up on top of itself:



Then select the profile curves, and run Construct > Sweep and pick the spiral as the rail to generate this:



Draw in some lines and trim the spiral so you can form this planar region:



Select those curves and run Construct > Planar to fill that area in:



These can now be joined:



Looks like this:



- Michael