Modern Linux DPI support is a nightmare. It's a shame, since if you just run and old-school software stack (X11; minimal window manager; xrandr to adjust DPI if you hotplug a monitor), then it has much nicer font rendering than Mac OS.
This is particularly frustrating since I've been using high DPI displays since the CRT days. Everything horribly regressed about a decade ago, and still isn't back to 1999 standards.
IDK, high DPI worked fine for me under Linux. I just set the desired DPI in Xfce settings, and everything scales properly. (Except Firefox, which has its own DPI setting! But it works equally painlessly.)
Where things go haywire is mixed resolution. It's best avoided :-\ Hence now I have a 28" 4k external screen which is exactly like four 14" FHD screens of my laptop, so the DPI stays strictly the same.
This is particularly frustrating since I've been using high DPI displays since the CRT days. Everything horribly regressed about a decade ago, and still isn't back to 1999 standards.