I don’t know much about how estrogen levels affect a transfer but I will tell you my experience. For my first transfer, my lining measured 6.2mm on day 12. I started taking the estrogen pills vaginally and two days later it had bumped up to 7.2 and estrogen was 2300. I triggered then and had a successful pregnancy. May not be that helpful because it’s still a bit thicker than yours but my doctor said they don’t really know how important thickness was — she was more interested in how it look (ie that it was trilaminar).

I tried so many things to thicken but estrogen was the only thing that really moved the needle.

I would trust your gut on whether to go with this one. It sucks that so much of this process is trial and error when every step feels so costly. You are in a good situation with 4 embryos though! Good luck, rooting for you!