I went as far as I could with the tee shirt muslin. It was mostly a success. I've verified that the pattern fit, and I identified a few things I could do at set up to make it easier to put together going forward.
I do have to revisit hemming. There were some tricks in the video for how to do the sleeves, as well as how to do both sleeves and bottom hem without bunching. (And I was using the walking foot, so that wasn't my biggest problem.)