I think you should get the dress & wear it if you love it op
Im in my 50s & have been to many weddings over the years & i have never ever seen anyone arrive in jeans & a top!!
Nor have I ever seen anyone apart from the bride change outfits for the evening event. And even then that's been rare in my experience
People are dressed for for the wedding & stay dressed!
If that dress is full length on you perhaps have it taken up to ankle length which would make it less bridesmaid like
But I think it's entirely suitable. I think jeans & a top no matter how fancy would be all wrong!
Congrats on the weight loss - that's a fantastic achievement!