You are making this way more complicated, than it needs to be. What is the goal, to learn to hone, or to compare grits from unknown grit sources?

Simplify, and use only known quantities, remove all unknown variables. If you don’t know a stones grit, don’t use it.

First learn to hone using quality synthetic stones, of know performance and grit. Once you have mastered honing and understanding what you are looking at and know that the progression works and will actually hone a razor, that you can shave with, then add natural stones to the mix and compare results to what you know.

Start with a quality hollow ground razor that will take an edge. You can buy a good razor from eBay, any antique store or in the classified for $20 or less. If you try to learn to hone on a, difficult to hone razor, how will you know if a problem is the razor, the stone or your technique. Eliminate as many variables as possible.

All diamond paste is not alike and different paste will perform differently on different substrate. In short eliminate all the “Diamond” strops. Any thing larger than .50um, 20-30,000 grit is too large for razors and if the diamond is not quality, can lead to a micro chipped edge. So, for now eliminate the diamond. BTW, 400 laps are way too many laps. It is impossible for you to have done 400 pasted laps and there to be no change to the stria pattern. Keep better track of your photos.

If you are going to make photographic comparison, photograph the same area each time, mark the bevel so you have a registration mark. Wipe the bevels with a clean paper towel and a dry microfiber. This will eliminate all the horizontal swarf marks on your bevel, so you can better see the stria.

Which quality synthetic stones do you have available? And what is the goal?