I tried MyST recently. All I see is a markup language that slowly become more and more complex over time to support more and more features that LaTeX already supports while at the same time acquiring the same syntax complexity of latex.

What people don't acknowledge is that there is a base level of syntax complexity needed to produce fully general documents. If you do, the natural conclusion is that to fix latex, you need a full rewrite of latex with minor changes to fix all the inconsistencies that have crept into it.

My PhD written in MyST is here:

https://phd.row1.ca/phd/introduction

Allows web-first, as well as a PDF output (via LaTeX).

Going by the documentation it does it by... drumroll... converting to LaTeX!

(edit: generating PDFs that is)

To be fair, there is no better free tool than LaTeX to typeset PDFs. But it fails at non-paginated, free-flowing content.