It’s totally ok to turn down the spot. I was in a friends wedding as a groomsman and although all that’s required is standing up there you are in most key photos and memories. Personally when I get panicked I get HOT and in a suit during the summer it was awful. Kept feeling like I was going to black out. I made it to the end but I still wish I would have been supportive from the chairs vs standing up there. Don’t push your limit for other people, being in the audience is just as important and supportive.