External display doesn’t fill screen
Something is odd with the display - I expect you to have the traditional larger text / more space controls rather than a list of resolutions.

The resolutions are poor for controlling text size as you correctly have diagnosed.



Your OS is totally up to date and your hardware is well capable of driving that display. Does the vendor have an update to the display / firmware you might load to assist in working more like most display port monitors? Your Mac can use an iPad to run display, but since you already have this hardware - I hope my start to an answer either helps or someone can provide the clue / missing step to get back to the controls above.

One hint - when you are in the displays preference pane and your DUEX is connected - hold the option key and down by the help question mark - bottom right, you should get a button to "Detect Displauys" - press that once and exit out of displays / re-enter it to see if that helps get you different choices.

I would also check "Show mirroring options..." so you can see if the menu item for the second display helps you mirror / change the resolution.