In case someone has a similar problem:
1) I downloaded the 7-day free trial of adobe photoshop (in my case it was a one-time thing)
2) I selected [Image > Image Size] and changed the dpi from 96 to 300 (with re-sampling unchecked)
3) Saved my file to a .tif format
I was going to say Blender in a comment but you've mentioned it already.
Ease of use is most important. I tried blender and was blown away by the interface.
Blender can also be run almost completely with Python, if you are comfortable with Python then you almost never need to use the GUI or touch a mouse, which I also find intimidating.
update: I ...