Mods:

False seam (purl stitch) added between front and back, which improved the fit for me, making it less boxy.

Because of the gauge I obtained with my chosen yarn, I did some knitting math and found that knitting size L would yield measurements between M and L size which were just what I wanted. So I knitted size L.

Because of this, the neckline would have been too droopy with the number of stitches for size L, so in the first row of the ribbing, I reduced the number of stitches from 132 (size L) to 117 (size M) with evenly spaced P2tog decreases. I also knit the neckline on a needle one size smaller (US 5/3.75 mm) than the US6/4.0mm I used for the rest of the sweater.

There are many little design details in this pattern which are quite beautiful (e.g. specific decreases, construction techniques) so it pays off to read the pattern very, very carefully, consult the glossary of stitches, wrap your mind around the construction, and highlight the information for your size.