I've just looked at all the links you've shared and you've got really good taste, whichever you choose, you're going to look stunning.
I'm an experienced last minute buyer! (ADHD, crap at organising stuff, fucking great in a crisis).
OK, so a month is plenty of time. First of all, take a deep breath, this is going to be fine. (See good taste comment above!).
What you don't have time for is buying one outfit, trying it on and sending it back, then ordering a new one. You need to order at least 3 you like, try them on, choose the one you love and send the rest back.
Have you got PayPal? If money is an issue at all, that's really useful for buying things to try on at home. If it gives you the option of Pay in 3, go for that, then you only need to pay a third of the price, try it on, and then you can return it if you don't want it, before a huge chunk of money comes out of your account.
Also, women are still wearing white because it's a fashion started by Queen Victoria. www.vanityfair.com/style/2018/04/queen-victoria-royal-wedding
You say you're an atheist, I'm going to hazard a guess you're not a royalist either?
The dresses you've linked are lovely, but don't feel you have to stick to pale stuff. Women used to get married in their best dress, it didn't have to be while or pale.