Hey there, can’t even give you a proper like as apparently I’ve exceeded my daily limit 🙃
Anyhow that’s a really cool study. I think the changed results could be from jpeg artifacts, either save as higher quality or save as png and see if that’s what caused the issue. For diagonals, if using a brush, you can use the same brush, change to another color or background color and paint out the part you want to wliminate. Otherwise, a super easy way is using lasso tool and clicking on points to make shapes then filling in them. For speckles you can make or search for a “speckle brush” if you need some speed
Cheers