Its been about 5 years since the last time I was job hunting, so things might have changed. I did that email after rejection a few times, but the only reply I ever got was a "Sorry, we can't discuss that."

Someone on reddit mentioned that they straight up asking at the end of the interview, so I started doing that. I did that at my current job, even.

During those final moments of the interview when they go "I think thats all we need, do you have any final questions for us?" I would ask something along the lines of "Based on this interview, what would you see as my largest opportunity for improvement to be a better fit for this role?"

Try not to phrase it in a negative way or in a way that would make them highlight downsides, like "Is there anything about this interview that makes you think I'm not suited for this job?" The last think you want is for them to leave with the most negative aspects fresh in mind.

Obviously theres a risk of sounding like you already feel like youre not getting the job, but it depends on the recruiter and any reasonable person would view it as you looking for opportunities to improve yourself, and that you're willing to go the extra step in filling their need.