I have the sister to the Dell U3219Q. It's the newly released Dell P3222QE which is very similar to the U3219Q. When directly connected to the Mac Mini M1 via USB-C (cable provided by Dell), it will show 3008x1692 hiDPI at 60Hz. However, if I add a dock (OWC Thunderbolt Dock) it'll only allow 3008x1692 hiDPI at 30Hz. I could get back the 60Hz by changing the Display USB-C prioritisation to Display Resolution on the monitor itself.
What I am curious is, is it possible to set up a HiDPI at 3360x1890 for this monitor? This would mean creating a custom scaled resolution of 6720x3780 (slightly higher than 6K) in an app such as SwitchResX. I have no success in doing this.