Flexbox support has been [included][1] since 2018, although my use case was the prototypical one - a single row w/ 3 columns - so YMMV with how it handles more complex layouts.
In addition, so far I know, the @page "locations" (e.g, @top-center, etc.) are not well supported yet in all browsers which means the flexbox fallback is probably as close as we're going to get.
[1]: https://github.com/Kozea/WeasyPrint/pull/579