Amateur thinning and polishing questions

Hi all, I just tried to thin a knife (135mm blue 2 petty) for the first time (pictures below) and I have a few very basic questions. As said, I'm quite the amateur so please go easy on me :O

Before:


After JNS 300, 800 and synthetic red aoto:


As you can see there are still (a lot of) scratches left. Should I progress to a finer stone or use some kind of other polish like Flitz to remove these scratches?

A couple other questions I've been having are:
1) how do you know if you have thinned enough? Do you just eyeball it? Or try to cut something during thinning?
2) what is the best way to make sure each sided is thinned evenly?
3) thinning did not affect my edge, which was pretty sharp before I started thinning. Is this normal?

Let me say first that I am a fellow amateur and beginner who has only thinned and polished a few knives, so this is not necessarily what you should do but only what I do do (haha, doo doo).

0) For polishing after attaining the desired geometry I haven't yet achieved a consistent result with bench stones, though people certainly do. I use fine grit wet/dry sandpaper and I use it wet. Micromesh pads are pretty popular but I haven't gotten as good results with them yet. See the safety note below!

1) I eyeball, take measurements with calipers, and test cut. The last time I did one I matched the geometry to another blade of similar size and profile, so that was largely measurement-based, but if the goal is more qualitative -- "not so thick, with the edge bevel not so big" that might have to be visually based. If the goal is based on cutting performance and feel then you'll have to do some test cutting, but get this sorted before polishing (safety note).

2) For me so far, eyeball and frequent spot checks with profane muttering. I'm trying to devise a more concrete method using straight edges and measuring tools, and someone here probably has one.

3) Yes, because unless it's a single bevel with no microbevel or a Scandi grind, you're not necessarily thinning all the way to the edge. This might happen anyway and isn't necessarily bad (although if you reach the edge be aware that you're losing height) but for example, thinning a wide bevel knife that has gotten pretty thick behind the edge won't affect the edge itself.

Safety note: polishing a sharp knife with something like sandpaper or pads is dangerous. I tried once, cut myself twice inside ten minutes, and realized that I ought to intentionally dull the edge first, get the finish I wanted, and then sharpen only after everything else is finalized. Also be aware that when sanding or grinding near the edge you might end up accidentally making it sharp enough to give a nasty surprise.