Major progress has been made! Since I completed the unitard, I made a duct tape dummy, created pattern slopers, created patterns for the dress, and made a muslin "practice" dress.
The duct tape dummy was necessary, as I don't believe there is any dressform in the world that can be adjusted remotely close to my body shape. This was dirt cheap to create (plastic wrap + duct tape + fence post expanding foam) and didn't take much time (thank you to my husband for helping!). This dummy made taking measurements and fitting the slopers super easy.
I learned how to create my slopers from the book How to Make Sewing Patterns by Don McCunn. The pattern drafting videos on Creative Bug were also very helpful. Basically what the slopers are are generic, fitted patterns that you can then use as a base for pretty much any pattern you want to create for yourself based on a series of measurements you take from you body (or duct tape dummy) and a pretty lengthy (at least for me since this was my first time) fitting process. The bodice and sleeve patterns were very simple modifications from the slopers - just add seam allowance and sleeve fullness. Then for the the skirt panels, I went off of a tutorial for a Hobbit-style full gathered skirt I found online that suggested a skirt circumference of 3 to 3.5 times your waist circumference. I made some panels based on this (panel width = (3 x waist circumference)/4) and sewed them to a makeshift belt to try on. The skirt looked a little too full and the panels too wide compared to the comic images, so I cut 4 inches off the width of each one and was pretty happy with the result. So that makes the final ratio of the skirt circumference to waist circumference 2.5. I made each panel long enough to go from my waist to almost the floor when barefoot so that there would be plenty of wiggle room for hemming.
Before I make something that I developed myself, I like to do a test run on muslin to validate my pattern (or pattern modifications) and process. Yes, it took a full day yesterday and a few hours today to make a dress just for practice, but doing this brings my anxiety way down and my confidence up before going into the real thing. I'm pretty happy with how this turned out (except for looking like I belong on Little House on the Prairie), and I'm ready to start the final dress! Note: the bottom of the center back is a little funky and has a small gap because we just pinned it shut. This should be better on the final dress with the zipper.