FWIW re-solving 21 feels like a bigger deal than it actually is. Partly because your memory of the level tends to be shaped by how long it took to find the solution rather than how long it takes to implement the solution, and also because 21 is pretty generous space-wise, so the solution's a bit more forgiving to memory lapses.
I was delighted to get another level. I haven't solved it yet, but I'm 99% sure what I need to do with the green target, and have gotten (probably deceptively) close to a solution. And if it's what I think it is, then it seems like the perfect conclusion to your little puzzling monument.