An alternative algorithm which would probably converge faster than 100 questions would be something like Elo or Glicko 2.

A word's "difficulty" would be some function of how rare it is. Once you have a reasonable estimate of the user's "skill" you can infer that a user won't know more difficult words. The benefit of this is you're not spending time asking the user about words they probably know.

Of course it's possible at an individual level, difficulty does not monotonically increase as a function of how rare the word is. A person might be very familiar with a domain-specific subset of English. But the "stratified sampling" approach will also have this problem.

There is a similar problem in chess, where players have ratings which really only change on one dimension. So there can theoretically be a mismatch when puzzles are also scored on a single axis, since a "harder" puzzle that contains a motif a player is familiar with will actually be easier for the player.