Logically having your OS maintain a consistent UI size makes sense until you try it without.

I'm running a couple medium high density monitors alongside one of the highest density ones available. I don't scale the HIDPI monitor at all, which means when I drag windows to it they are tiny. Instead it works in two ways, as a status screen for activity monitors/etc and as a text/document editing screen. AKA putting adobe acrobat, firefox or sublime/emacs/etc on the high DPI screen and then zooming in gives all the font smoothing/etc advantages of high DPI without needing OS support.

So the TLDR is, turn off dpi scaling, and leave the hidpi screen as a dedicated text editor/etc with the font sizes bumped to a comfortable size. Bonus here is that the additional effort of clicking the menu/etc will encourage learning keyboard shortcuts.