By the time an interview ends, a candidate usually has a good sense of whether it went well or not. I've interviewed hundreds of candidates and I would never jump into this feedback mode OP describes. There's not much point in giving positive feedback if they're already feeling good about the interview. If you give someone negative feedback (no matter how constructive) they're going to get defensive and want to plead their case; but it's the end of the interview and there's no time for that. OP says people express gratitude for the feedback, but what does he expect them to do? Anyone who doesn't appreciate it still doesn't want to tank the interview by appearing ungrateful. Never play psychological games in interviews.

The problem is the interviewer though.

There is no fixing an interviewer who doesn't understand the answers really. You need a third party for that.

Have had weird interviews with people who don't introduce themselves and assume that I definitely am interested in whatever the job is even though I know nothing and proceed to ask questions that amount to "how does palantir work" and get irritated when the answer isn't "we used database X".

Fundamentally one needs a meta approach that handles these cases too without knowing who is interviewing who. Could an interviewee give feedback like this without sounding arrogant?