I personally use Calibre+Calibre-web.

It’s configured as a proxy for the Kobo store, the default store for my e-reader.

That means that when I click the sync button on my Kobo, it downloads anything hosted on my calibre-web server, while still keeping the ability to browse the Kobo store.

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.