I am a fellow eventer and also made my debut at USHJA shows this summer!
I don’t know if you already have these ducks in a row, but make sure your horse has a microchip. I know you said you read the rules, but that is a notable difference since we don’t have to have them until FEI in eventing.
At the AA show I went to in May, I showed Fri-Sat in 1.10 adult am jumpers and Sun in 1.0 low adult am jumpers and everyone wore a white shirt w/ buttoned collar (no stock tie), tan pants, and a coat. The rules only specified proper attire for the 1.15 classic (which I scratched out of), so I assumed the default for everything else was “standard”/I could’ve worn a polo with no issue, but I went ahead and wore the coat/matched everyone else. People in the schooling classes were fairly evenly divided on coats vs. polos.
At the B show I went to this past month, most people wore polos and a few wore coats. I wore a polo for my 1.0m and 1.10m classes. At least one person wore dark breeches, which shouldn’t of been allowed as per the rules, but I didn’t see them get reprimanded.
There was definitely a bit of culture shock for me. Be prepared to waste your whole day. The shows I went to used horseshowsonline.com for entries and would post class counts the night before- it seems pretty normal that you can add/scratch classes freely up until the day before. You can estimate 1.5-2min/round to estimate times, but it’s only an estimate. I found out on the last day of the May show that they used shownet.biz to post estimated time schedules/drag breaks.
Bring your husband to set jumps. People are a bit ruthless about “claiming” jumps and you’ll get really dirty looks for just jumping in on a jump, even though it happens to be set at the height and width that you need. Or at least that was my experience. I found that it was just better to have my ground person grab a fence once it was open, stand by it to “claim” it, and then set as needed for my warmup.
I did enjoy myself and it was definitely nice to get to go in a ring multiple times, and I loved that all the fences were dressed up. I’ve experienced some rude/clique-ish-ness at schooling jumper shows before but everyone at the rated shows was either benign or friendly. I wasn’t scared away and am actually planning on one more A show next month before AECs. Good luck and have fun!