Emacs with org mode, helm, and projectile as a starting point would hit all of your asks (although org syntax has differences from markdown, it's still quite intuitive).
I use org mode for: my tasks (gtd style), food log (using org columns), notes (easy export to any other format), habits. Alongside beorg for iOS for quick capturing on the go, I cannot imagine I would be any happier elsewhere.
Same here, org-mode is going nowhere, it's one of those things that just works. From wikis, todos, document authoring, agendas, code notebook, ...
On mobile, there's a few options, I made one, a web app called filestash: https://demo.filestash.app/s/hn . It has a lot of the org mode candies: agenda, todos, and the real org-mode exporters: HTML, PDF, Markdown, TXT, Latex, iCal, ODT and even beamer
I use org mode for: my tasks (gtd style), food log (using org columns), notes (easy export to any other format), habits. Alongside beorg for iOS for quick capturing on the go, I cannot imagine I would be any happier elsewhere.