You have literally said this:

> Windows cannot do custom dpi per monitor, only a single custom dpi that gets applied to all monitors.

Here are all of my monitors at different DPIs: https://imgur.com/a/q3z2P1E . They don't have a "single DPI" that gets applied to all of them. The custom DPI setting is for changing all base system DPI.

> I cannot set one monitor to 120% and another to 160% (both are custom values), like on KDE.

Okay you're unhappy with the granularity. Yes Windows uses 25% granularity.

I don't know if this will work but you can probably do a combination of changing the base DPI and then calculating the 25%. So you can set the base DPI to something like 120 (which is 125%) and then set the other monitor to 125% which gives 156%:

I think the base DPI is stored in this registry key:

HKEY_CURRENT_USER\Control Panel\Desktop\WindowMetrics\AppliedDPI

It is a DWORD value