Simplest Future-Proof Note-Taking App

If I hadn't built Flux.Garden, and I didn't care about working in public, and I was a 25-45-yo time-shifted version of myself, what would I use as a note-taking app for my private digital garden?

I think I would require

• markdown
  • this way, even if it's not browser-based, I have links to get to other browser-based objects like whiteboards
• Double-Brackets for linking, backlinks
• files not db-entries
• mobile "friendly"? What does that mean?
  • automatic/real-time-ish data synch (put it in dropbox?)
  • links/backlinks work (e.g. not good enough to limit mobile use to reading/editing as legacy/pure markdown files)
• I'd have a definitely preference for open-source (esp in Python), but those previous bullets might give me enough safety/comfort that I could relax that....

Now that Obsidian has a good mobile app, are there any other good alternatives?