I have a few need-to-run apps that do not scale well to a HiDPI screens (the app window itself scales properly but all the text and buttons are tiny and in worst case unusable). For instance AnyDesk. Is there any built in solution for this in Zorin OS?
I know there are some iffy solutions for Ubuntu, but again, how much does one really need to hack for basic functionality? 
Are the apps that are not scaling well QT-based apps?
You can export to your profile:
QT_SCREEN_SCALE_FACTOR=
Where = is followed by the scale factor.
Or you can use xrandr scaling instead of HiDPI.
https://forums.linuxmint.com/viewtopic.php?t=159064
So the answer is "no" to my initial question: "is there a way without a hackaton"
I highly doubt they are QT-based but will look into that.
Compile a separate xrandr? Is that really the only way?
I give up. I'll just reduce the screen resolution to something not-HiDPI. That's the easy but disappointing solution.
The answer is 'yes', because I would not consider adding one line to a file to be a "hackaton."
Glad to know you solved it fitting to your standards. 
Try this:
xrandr --output HDMI-1 --mode 3840x2160 --scale 0.7x0.7
Or change 0.7 with another smaller value to get a bigger size.
That post is quite old so I think that things now only require a single line:
xrandr --output HDMI-1 --mode 3840x2160 --scale 0.8x0.8
where HDMI-1 is the name of the screen which can be found by running xrandr without any parameters.