You need over 90% enriched for practical nuclear weapon use. It is not "most of the effort" to get to 20%: in fact, if you're stealing it then it's none of the effort: you don't have a centrifuge operation, so whether it's natural or 20% or 70% it is all equally useless.
The difficulty of enrichment is the plant and logistics of doing it, none of which you can steal.
I should have been more clear that I am assuming an hostile nation-state with some minimal capabilities : the biggest difficulty is probably to be able to deliver the bomb to a target once you have one, which gets even worse if you want to use it as an open threat for deterrence rather than a (frankly suicidal) surprise attack : you pretty much need an ICBM (see all the North Korean failures before they (supposedly) stole SSR-Ukrainian tech).
Difficulty of enrichment seems to be heavily proportional to time : the whole "game" around Iran's enrichment seems to be about not letting them get it high enough that they can go from weapons-useless uranium to a ready bomb in a short enough amount of time for the other countries not being able to react.
(There's also a possibility of a much smaller actor to make a dirty bomb from that mildly enriched uranium, but I have even less ideas about how likely that is.)
The difficulty of enrichment is the plant and logistics of doing it, none of which you can steal.