all I included was my passport, my birth certificate, my W2 from last year, and a few recent sequential paychecks. I was approved.
I don't think the standard of evidence is extremely high, in case anyone was wondering. I make a very average wage for my area, maybe even a little below.
also impatiently awaiting the email with further instructions. very happy the first step didn't take a ridiculous amount of time.
edit; I will say that I specified exactly what kind of support I am going to give my partner, that may be a factor (since we're getting married ASAP anyway, I just stated that she's going to have full access to my income).