Scaling UI - Resolution change makes it blurry

There is another thing you can try -- enable the Accessibility System Preferences pane to set the Zoom option "Use scroll gesture with modifier keys to zoom" and set the desired meta key (I use option) -- then hold down that meta key and use the scroll feature of your mouse to zoom in and out (what kind of mouse are you using).

Also suggest clicking the "Zoom follows the keyboard focus" so that you can zoom into wherever the screen cursor is currently located. Also suggest enabling the "Use keyboard shortcuts to zoom" and "Smooth images" setting also.

It takes a bit of getting used to, but not too much. It lets you quickly zoom in/out as desired but at the same time gives you full access to your high res display's full features. Unfortunately all three monitors will zoom in/out, so that's the downside. Perhaps there is some third-party utility that will let you zoom in/out on a single monitor?

Good luck...