Have you tried Prince? I haven't used it myself, but heard good things about it.
Aren't margin boxes the way to go for page footers and such?
edit: This page recommends PDFreactor, which seems to support at least flexbox https://print-css.rocks/tools.html#pdfreactor
Neither can you really use min-height=20mm. A pity. And the nicer pdf creators (weasyprint) don't understand the new css rules like flexbox or grid.
So we are still stuck. No easy way to force a footer to the end of a printed page via css.