Then fix the timing, because a month late is the real problem with your plan.
Ask at the moment they leave, automatically, from now on:
When a trial expires without converting, one email, one question. Not a survey, a single question you would answer yourself.
When somebody cancels, one question on the cancellation screen. Free text, optional. People are surprisingly forthcoming at that exact moment because they have just made a decision and it is fresh.
When somebody stalls at a known step for a few days, one message asking whether they got stuck. This one frequently converts people as well as informing you.
On the wording, since that is what is stopping you: the version that gets replies is short, personal, and asks for help rather than feedback. Something like, you signed up a couple of weeks ago and did not carry on; I am trying to work out why, and one line would genuinely help. Nothing to sell in it.
Expect a low reply rate and useful replies. Five out of thirty-eight is a normal outcome and five is plenty.
And do email the thirty-eight now. Late is worse than immediate and much better than never, and you will not have another cohort for months.