First of all, thank you, worldbuilding pasta, for the very useful tip. As it turned out, I WAS using the T21 topography input files when I should have created a new set for the T42. I'll be sure to remember this in future simulations.
Thanks also for explaining the issue with the precipitation; I'll definitely make use of that script you referred to.
Here's my latest progress for my first continent, with and without the texture map.
In case you're interested, I used Wilbur and Photoshop to create these images.
To all experienced cartographers, please let me know if I'm anywhere close to my goal of rendering realistic topography. I'm considering using a real DEM in case the vote is a "no."
Thanks!
Peter