Urgent XPS 15 (9530/9540): Inconsistent High-DPI Scaling and Blurry Text on External 4K Monitor

I'm encountering highly inconsistent UI scaling issues when using my Dell XPS 15 (9530/9540) with a docked external 4K monitor. The internal display is set to 300% scaling, and the external is at 150%. At the same time, most native applications adjust, non-DPI-aware apps (and some specific older utilities) display with severe blurring or are microscopically small.

The core issue appears to be a conflict that occurs after unplugging and re-docking: Windows frequently reverts the primary display's scaling or applies a mixed DPI state, requiring a full reboot to resolve. Has anyone found a reliable registry fix or a specific Dell/NVIDIA driver setting to force consistent pixel-to-display rendering ratios, especially for applications that are sensitive to the root scaling change?