Every place I have worked at has remained seated for the "standup" because it's tiring to stand for that long.

(For those who aren't looped into the irony: the entire reason why it is called a "standup" is because you are supposed to stand up to create subtle pressure against long unnecessary meetings. Sitting down for a stand-up isn't just a quirky paradox of jargon-meets-reality, it is a fundamental rejection of the entire premise while cynically parroting the terminology itself.)

Exactly.

My stand-ups are 10-15 minutes, usually. When they use the full allocated 30 minutes it's because somebody wanted to dig into a specific topic (and anybody not interested in that topic is free to leave).

This is a normal agile development team - 1 manager (me) and ~6 engineers. We do the quick round-robin update, I'll make any announcements or ask for clarification, then if there's something that requires it, a deep dive (as deep as you can get in the remaining time).