The side-by-side view doesn’t do it for me, I’d more likely than not have multiple windows open with different documents instead.

That’ll probably rule out text editors like emacs tf you don’t want side-by-side. Emacs has some functionality that can do some styling, but you probably won’t have a purely WYSIWYG mode for, say, tables. It looks like emacs has some way to translate org-mode tables to Markdown, but that’s probably not quite what you want.

It should do autocomplete, syntax highlighting, bracket closing, live spell checking in a variety of languages, launch quickly, be rock solid when faced with a massive log file and allow me to add menu-items to run bash scripts that do things like calculate the time it would take me to read out the text at my normal podcast reading voice or covert weird characters into hrml-entities.

That’ll rule out most “small” programs targeting specifically Markdown.

Depends on what you mean by “massive” log files. If you mean you require out-of-memory editing – the ability to load only a small portion of the document into memory, which is probably going to be necessary once you exceed your machine’s main memory – then you’re looking at a small set of software. Some hex editors, emacs can use vlf (which will constrain other features available), a few programs targeting specifically this feature.

I haven’t looked at heavyweight word processors, but some may have reasonable support for at least many of those, stuff like LibreOffice. They probably won’t open quickly, but there are a few programs capable of speeding up startup by leaving a daemon running, just opening something in that daemon, like emacs, urxvt, etc. You can possibly do that or just leave a blank document open on another workspace.