Probably better not to. From my experience tabs (in the long run) are more annoying than helpful. Take vscode: I use the "open editors" menu, because tabs work just up to 3 of them. Then they either not fit (scroll-x), have titles far too short to be identifiable or take half the screen when using tab wrapping.

I haven't used heynote yet (in a train rn), so you may've that already in place, but:

I suggest implementing bookmarks with fuzzy search. Press ctrl+b a prompt comes up, type the thing, press enter and get your file scrolled to that section (markdown title)

Right - not sure tabs is the right metaphor. I definitely want the ability to keep some blocks around, put them aside, return to them later...

Maybe a block 'tagging' mechanism? Let me 'stash' a block and label it with tags... then later on I can 'restore' stashed blocks by searching for tags. Or open a new window easily containing every block that shares a particular tag?

Perhaps you could consider documenting/specifying the file format. Seems to me that this could potentially become some kind of standard (if you're open to that). Perhaps with a .mixt extension (for "mixed text")?

Seems to me that this would be a pretty good general document format for a lot of use cases.

I think tabs could be interesting if coupled with a way to filter the list of blocks (ex: I only want to see math blocks, or only blocks containing the #daily tag, or only blocks from this month).

Then each tab would be an opportunity to apply a new set of filter on top of the same document.