Dex mode should deliver truely desktop experience. However, the app title bar makes it worst. There should be an option to hide this window or should have an option to enter exclusive mode to get rid of taskbar and title bar.
Try going into settings, Samsung DeX. Go to the eighth option and that hides the task bar. I'm using Tab S7 plus & accessing settings while in DeX mode. There's an app in the Google Play Store called DeX Max this can be used to hide the title bar, I used to use it back in the day but it would stop working occasionally so I uninstall it. I don't know if it's been updated since then, but it's worth a try cos it's free.