Grid everything, it has everything flex has and mixing the two makes one want to rage quit.

I've struggled with children sizing in flex. Children take up more space depending on their content. If the content overflows, flex doesn't handle that well either.

Grid seems to handle this better when you specify grid-template-column.

Anybody know a good flex-specific solution?

Maybe this: https://drafts.csswg.org/css-rhythm/

The searchable keyword for that is "responsive".

Using length units related to viewport size on max-widht may help.