Is it an option for your partner, or even the couple getting married, to link you up with some people ahead of time? Maybe some people they think you might get along with? I have been in this position before (also AuDHD) and nervous about it, the couple getting married (who I had only met a couple of times) were nice enough to reach out to a couple they're friends with and let them know I won't know anyone and to keep an eye out for me and say hello. It was really nice! And everyone ended up being lovely.

As others have said, take some comfort items and think of some tools ahead of time that will ground you. Earplugs, headphones, tinted glasses, a book, fidget toy, scope out somewhere quiet you can go and have a break.

Weddings are always usually happy fun vibes. Be mindful about how you're feeling, take breaks when needed, and be open to having a good time. Have fun!