So you're saying is:
Change to 1080p resolution and 125% zooming.
Log out.
Log in and change back to 2160 and 150% zoom.
Log out.
Now whenever I'm in either of those resolutions I'll have the correct zoom?
One more question: Make the changes in Windows 10 settings or Nvidia? I think the zooming is only in Windows settings.
You want to keep one zoom level so that it is not changing with the resolution.
Though things are improving, changing the zoom level without logging out and back in can cause scaling issues for many applications.
In my case, it was a slightly different issue.
I use NVIDIA's DSR to render games at resolutions like 9976x4176 for extremely high image quality on a 3440x1440 display.
The problem is that when selecting a resolution that high, Windows was setting the scale to 300% which broke a lot of things without logging out and back in.
The solution was to set the desktop to 9976x4176 (rather than the game) and set the scale in the Windows display settings to 100%. After that, whenever I changed the resolution in a game, the scale stayed fixed at 100% for everything below that resolution and there were no more scaling problems.
So if you typically want to run 3840x2160 at 150% scale, you should also set 1920x1080 to 150% scale so that the scale does not change when the resolution changes.
I'm not sure about switching resolution for 3D content.
Is there a reason you are wanting to set the resolution to 1920x1080 for that? I thought that side-by-side rendering (1920x2160 per eye) resulted in better image quality on 4K displays - especially since it bypasses the 24Hz limit.