Thanks to all for the helpful feedback. I think we'll stick with the tight window, book the FastTrack through immigration, and hope they're efficient with the bags. I did look at this flight history and it generally arrives between 5 minutes late, and 30 minutes early--so that's good.
Yep, I second Real. I've got a decent touring helmet that's safety certified, relatively comfortable, and was only like 2-3k baht. The shape may not work for all so you'll want to try to fit first if you can.