Hi Frank,
The Surface Book 2 can support different monitors with up to 4096 x 2304 screen resolution for a one-external-display setup (Surface + external display). It is true that the Dell monitor you have is not known to work well with your setup with the Surface Book 2 and Surface Dock. However, applying different settings and updating both the Surface and Surface Dock should give you the best output on the external display.
To get the recommended driver and firmware updates for the Surface Book 2, download and run the Surface Diagnostic Toolkit (SDT). Keep the Surface Dock and external monitor powered on and connected to the Surface. After running the SDT, perform a forced shutdown by pressing the power button for about 10 seconds. Restart the Surface normally.
Next, use the Surface Dock Updater Tool to install the best update for your Surface Dock.
Once done, see if you'll be able to get a higher output resolution for the setup. You can also try to apply a different refresh rate and see if it will let you choose a higher resolution for your Dell monitor. Go to Settings > System > Display. To configure the refresh rate, click on Advanced display settings > select Display adapter properties for the display > go to the Monitortab > choose a refresh rate from the drop-down menu.
If you are still unable to get the desired output after following the suggestions, I would suggest that you use the Feedback Hub app to raise this to our product developers. We will be working on the compatibility issue that may be the reason why the display is limited for your Surface and specific external display.
I'll appreciate it if you can provide an update by replying to this post. That way, I'll be able to check the situation and provide further help if needed. I'll be notified when you reply and I'll get back to you the soonest I can.
Regards.