hugo --help showed me that Hugo has a Jekyll import function.
Since I had those social posts that I eventually want to get into Mb, this ended up perfect. I ran
hugo import jekyl PATH-TO-JEKYLL . to transform those posts into Hugo compatible files.
I forked the marfa theme and put it into the Hugo folder. Error about a missing
custom_footer.html file. Pretty sure this is part of the default Mb themes, so I just created an empty one in partials, and the site built and served locally! Very fast compared to my Jekyll experience: 568 ms for 569 pages and 220 static files!
Looks like custom footer was the only one missing. I'll keep track of edits to it on the Marfa Theme notes page.
Enabled the OpenGraph plugin so I get social previews.
Added a Blog category to the navigation, for all the actual blog posts with titles, rather than just short social ones.