Why prime length is a late-game rule
Prime length is simple to check but hard to preserve. Every character you add or remove changes the length, which means a solved prime length can break immediately when you fix another rule.
That is why prime length is usually best handled after the major content requirements are stable.
How to tune length carefully
Once the password has most required pieces, count the current length and move toward a nearby prime number. Add characters that do not disturb Roman numerals, digit sums, or uppercase-vowel rules.
If one added character breaks another rule, undo it and choose a safer filler character. Small edits are much easier to reason about than full rewrites.