After way too many years of running my own web server, about half the time writing raw HTML or Markdown, I started using https://www.silvrback.com/ a while ago. It has a pretty nice editor with special consideration for posts including code or mathematical formulae, and without most of the warts the OP mentions at Medium. Might not be for everyone, but I find it quite pleasant.

Non-disclosure: I have no relationship with Silvrback other than as a plain old customer.