My daughter was in this same position. She didn’t repeat but I still think she should have. the academics were fine and she kept up fine, but if she’d been with the class a year younger, she would have breezed through. Yes, she was a little bigger when she was 14 (a sophomore) than she was at 13 and a freshman, and sports would have been easier for her to shine, but it was the social aspects that were the hardest for her.
As a freshman, she had no choice but to hang out with the other freshmen because there were no students younger than her in the high school. We moved when she was a sophomore (mid year) and almost all her friends were freshmen. Some were actually older than her because they had Sept or Oct birthdays while she had a December one. When her sport was in season, she traveled freely between friends in the grades, but when it was off season she migrated to the younger kids, kids her same biological age. When the older kids went to the R rated moves, she couldn’t go. They could drive but she couldn’t. I was more strict with a 16 year old (no prom party at the beach) than I might have been if she were 18.
I don’t think OP’s only reason for wanted to do a repeat is athletics. It’s ONE reason, but there are others too.