That fact that you were yak-shaving implies that you wanted to get certain details just right or you want it to function a specific way just for you. And that implies software with more functionality that you can tweak. So inevitably more complex software. If it really was '$script article.md > article.html', you could simply do that with your own domain -- just put up the html on your domain with a link to it (which is trivial to do)! Also, I think thinking about "small CSS/minimal JS" etc means also that you are mixing up concerns -- seems like you are too much concerned about the tools than about the writing...sorry, not trying to criticize, but got reminded of myself when I attempt to do something trivial and go off into the weeds thinking about the tools that get my job done instead of getting the job done. :-). The tools won't fix a writing problem and you should focus on the writing and see if they are worth the trouble finding the right tools for the job.
And to answer your question, wordpress.com, ghost.org, medium.com -- all of them provide simple publishing that meets your requirements (except perhaps for the 'minimal' part).