A couple of years or so ago, I managed to "solve" this issue in Ubuntu 14.04, if I remember correctly. I somehow figured out how to drive Unity at 162% scaling for my XPS 13 and scale it down to 100% for my external display. There definitely were some artefacts, but it basically worked.

I'm sorry to say though, that I don't remember how I did it, since I've been using mainly macOS and Windows for quite some time now.

Did that solution involve using xrandr scaling parameter? unfortunately that's not right solution IMO, as I observed it takes lot of CPU. Hacky and feels laggy.

My solution is to organize myself so that I don't have to move windows around. I mostly need browser, terminal and emacs. I open chrome in one monitor with specific scaling option, and when I move emacs or terminal around I manually decrease font size with Ctrl-+/-

What issues do you face, because I currently use that exact workflow with a MacbookPro retina, and a 24" dell monitor. Apart from windows not resizing/positioning automatically (so I have to manually re-layout everything whenever I switch from monitor to laptop), I haven't really suffered any problems so am curious to know what issues I'd face if I switched to Ubuntu.

I did face problem with macOS and having external low-DPI monitor: the Finder would act up sometimes: when you rename file the caret and edited text would be someplace else that the filename shown in Finder and dragging and droping files would stop working. Firefox had similar problems with addressbar and controls in web pages too.

All these things disappeared when I changed the external monitor for a HiDPI one.

I use a tiling window manager, so positioning is not a problem. Problem is that, on a my laptop (4k) screen things look normal, but on 2k external screen things look bigger by default. Ideally the GUI toolkit should be able to pick up per screen scaling factor.