> - future proof file format (markdown)

Trilium has Markdown-like editing experience, but it stores the notes in HTML. Which is IMHO more future-proof than Markdown.

> - possible as local standalone + sync to simple cloud storage (like gdrive/dropbox/...)

Trilium stores data in an SQLite database, and sync tools like gdrive/dropbox don't know how to sync it and usually corrupt the database. To sync correctly, you need to have a server with trilium installed.

No—Markdown is just plaintext, which by definition is the most future-proof format that exists.

HTML is a structured markup language that has multiple versions and tags that come and go. And it’s dependent on a user agent (such as a browser or voice assistant) to render it into something a human can view or listen to.

And while Markdown can be rendered, it’s not required to be human readable.

HTML is human-readable as well. I'm not sure if it's really a dealbreaker if you have to read <em>text</em> instead of em.

Markdown's big problem is standardization. It does have a standard, but it doesn't seem widely implemented, and it's notably poor - not even basic features such as tables are standardized. That's kind of a big deal for being future-proof. Worse, Markdown documents do not even record the Markdown flavor they are using.

You might say that it doesn't matter as it's still sort of readable even if you can't understand exact formatting detail. But again, so is HTML.

In my view, the future-proofness which actually matters is the tool support - and you have a much higher chance that in 30 years tools will correctly support HTML than Markdown.