Pu239 warheads are much harder to design.
Our advanced designs from the 80s to now all use Pu239 in the primary and U235 in the secondary.
I understand U235 is harder to compress so it needs more explosives in the primary and thus the warhead is larger.
Plutonium is more difficult, but both India and Pakistan have 75 warheads each of that type so how hard can it be.
Further, Pakistan has had those warheads 30 yrs.
It’s more difficult, but in terms of calendar, it is much quicker than spinning U235/U238 for years.