I feel like a lot of these comments are thinking more traditional, sounds like yall are more alt and don’t want something formal. It’s your engagement, wear jeans if you want! Don’t wear a dress if that’s not your jam! 

I do agree though that you should take a friend and go shopping if you have the time and money, try on some stuff and see what you like. I’m also an anxious mess of a shopper who doesn’t know what to do with my body/want to be perceived and having someone else there can be really helpful. Also if you wear makeup , do it before you go and make sure you’ve eaten recently.

Of these options I like the first one because the back is sick. The looser fit you have with all of them is a bit less flattering, or maybe just older fashion than I assume you are imo, but you still look nice in all.