I was looking at my mower and actually may be able to raise the height by
changing the front wheels, at least.

The back ones may adjust down low enough.

With the front ones, I measured the outside of the bolt threads with a
caliper and got about .33" diameter to the outside of the threads.

What is that, about 5/16"?

The present wheel is about 8" outside diameter, so if I can get one that's
10" OD, it will raise the height by 1".

Where do you find these things?