Pretty fun. I suggest skipping the submit button and just showing it's correct when pressing and moving on after a sec or so. Having to click on submit twice really breaks the flow. Also in all the words I tried I noticed out of the 4 options one is the correct one, another is the opposite of the correct one, and the other 2 are random stuff. You can basically skip any option whose antonym isn't present as well.
It estimated 74k words for me, but I feel this might be inflated; much of the time when I didn't know the answer - I could vibe guess it just as you did it. The distractor answers weren't convincing enough. For starters, when an answer was based on deconstructing the word into common English words, that ruled it out. After all, if it was, then it wouldn't have been obscure. A tangent: writing distractors for multiple…
In case of online quiz you can have a "competition" between distractors:
1. start by having much more distractors than needed and pick randomly
2. for each measure the probability of it getting clicked (clicks/times it's shown)
3. show the most frequently clicked distractors more often