The part about having around 250 people join the playtest but receiving only two public feedback posts stood out to me. I suspect this is not necessarily a sign that people were uninterested. Playing something is relatively easy, but writing useful criticism publicly requires much more effort and can also feel uncomfortable.
I am currently preparing a much smaller closed test for a narrative adventure, and this makes me think that relying on voluntary public feedback is probably not enough. My current plan is to combine direct observation with a short private questionnaire and a few follow-up conversations. Were you able to see where players stopped or how far they progressed? If the average session was around twenty minutes for a one-hour build, there may be a specific early point where people lost direction or motivation.