Enriching uranium is within the capability of a nation-state the size of Ukraine. If Iran and North Korea can do it, Ukraine certainly can. Hell, even today, in the midst of a war, they have 8 operational reactors at 3 power plants, plus 2 under construction, one damaged and recently repaired, 6 at the Zaporizhzhia power plant in contested territory, and 4 in the decommissioned Chernobyl plant.

Physical access to a nuke does not let you disable the PAL. They are constructed so that they are embedded within the device, and cannot be disabled or altered without deconstructing significant parts of the warhead. (I suspect that the PAL is not actually a separate device that can be separated from the warhead, but a series of design choices for how the warhead is constructed that make it unable to fire without the input of a cryptographic code. But then, details on this are very highly classified for obvious reasons, so we'll never know for sure.)

The real reason they didn't and don't do this is because they don't want to end up an international pariah state like Iran and North Korea. It's very clear that the U.S. has a vested interest in nuclear non-proliferation; they were the ones who gave the PALs to all our adversaries in the first place, because in the game-theoretic calculus of MAD, a small number of enemies that you can bargain and reason with is better than a large number of nuclear states even if many of those states are on your side. We would not have supported Ukraine if they attempted to retain the nukes in the 1990s, and we wouldn't support them developing nukes now.