It doesn't quite fit your requirements, but org mode from emacs is very close.

.org files instead of .md, and the preview does require a bit of config, but it's not as bad as some make it be, especially if you pickup a preconfigured emacs "distro" (like doom emacs for example) in which case I think it's just a feature flag to set to on.

Org is also very appreciated for it's TODO features, which you seem to make a big use of.

It probably isn't a match for you due to the markdown requirement, but I'm mentioning it just in case you didn't consider it in the past.