@thisdress1234 as others have said, honestly the ONLY thing that matters is that you like it and feel comfortable (in every sense of the word!) in it and that, when wearing it, you will still feel like 'you'.
I'm autistic too and got married in my early 30s. We had a really really small Register Office wedding - I wanted to elope but DH wanted his parents there, so we had to invite mine, so it became a 10-guest thing with a pub lunch afterwards. I was persuaded into a wedding-type dress, the £100 off the rack ones they used to sell at Debenhams and BHS by others who thought I should look 'correct' for my wedding. I was uncomfortable all day, both from a sensory point of view and because I felt like I was dressing up as somebody I wasn't. I didn't even know I was autistic back then but it still felt wrong.
Many years later I'm fully unmasked and 'out' as an autistic woman. My style now is very specific and I wear a variation of the same thing most days which is very 'me' and very comfortable! If I were to marry again now I'd be in something very comfy with a loud print and trainers/Docs on my feet. No make up. Just ME. I'd buy something new but something I would also wear every day afterwards, and that would make it special.
But of course the most special thing of all is getting to be married to the person you want to spend the rest of your life with. Wanting to BE married is the best start to a successful lifelong partnership. Wanting to GET married (big princess dress, 200 guests, etc etc) feels like a red flag to me although I know many will disagree and come onto the thread to tell me about their Princess Wedding and how they're still married 30 years later!
So you're already doing brilliantly by recognising what you really want and choosing to elope. It really doesn't matter what you wear as long as YOU love it and feel great in it. I honestly recommend dropping any ideas about what a bride SHOULD wear, even for a Register Office elopement-style wedding. In any wedding photos, the smile on your face having just married the love of your life will be what makes those photos into treasured possessions. It's not about your outfit.
Wishing you all the best for a long and happy marriage!