That's a really nice result. Using more value in brass than the chisels cost is quite funny, though. I use cheap chisels at work, spend time sharpening them, then some git pulls one out of your toolbag and starts attacking plasterboard with it.
I have similar chisels, and I'm just working on replacement handles for them. The only thing I've done differently is I've got rid of the ridge on the back (just in front of the ferrule) so you can sharpen the entire chisel without hitting it on the ridge.
This was a fun video. As you said at the start, my biggest concern was the quality of the steel. It looked like you could cut it pretty easily with a file which shouldn’t be possible with good quality hardened steel (as far as I understand). You mentioned you thought it was 20-30 rc which was surprising to me, is that Rockwell or some other unit? Good chisels are usually 58+ Rockwell from what I undertand. But when you used it, it seemed to hold an edge for at least a little while, so it must not have been that soft (or I really don’t understand the Rockwell scale). I think it was James Wright who did a really extensive chisel test and tested some Aldi brand chisels that actually performed pretty well, I’m not sure if it was these ones. But maybe the steel isn’t quite as bad as we would expect? You mentioned case hardening but I was wondering if you might try to retemper it and make them a bit harder. I dunno if that would cause other problems though, like making them more brittle and prone to chipping? Anyway, that would be neat to see if you try it someday.
I have a 5 piece set of Aldi chisels, but they have wood handles out of the box. I sharpened them similar to how you did and they’re fine. I wrap the full length of the blades with cardboard and put tape around that and to close off the end to make sleeves to prevent the blades from getting banged around and dull.