Ukraine never had the possibility to keep its nuclear arsenal, they simply didn't have the infrastructure for it, let's not pretend they had any real choice.
I agree Ukraine could probably build a new nuclear deterrent, similar examples as you give.
> They couldn't have launched the Russian warheads as-is, but disassembly and reuse of the warhead material is another thing entirely.
Another thing entirely, yes.
But consider that Ukraine build the Kuznetsov aircraft carrier, lost it in the collapse of the USSR because the (captain?) just sailed off with it: everyone would have noticed if Ukraine had tried to keep those weapons, and at least some of them would have demonstrated how upset they were with an invasion and/or by bribing guards to put them in trucks and drive them across a border (not necessarily the USSR-Ukraine border) regardless of what the government thought.
They couldn't have launched the Russian warheads as-is, but disassembly and reuse of the warhead material is another thing entirely.