In-Game Video Settings

Display Options
Display Mode: Fullscreen
Resolution: 3840x2160 (can't be changed)

3D Graphics Options
Display Brightness: 50%
Antialiasing Quality: 1
Post-Processing Quality: None
Shadow Quality: 2
Screen Percentage: 100%
Frame Rate: 60
VSYNC: On

Y -> Misc. -> Video Settings

Visualization options:
Visualization mode: Fullscreen

3D Graphics options:
Screen Brightness 46%
Smoothness quality: 4
Postprocessing quality: 2
Shadows quality: 2
Screen percentage: 100%
FPS: 60
VSYNC: Yes

Configuration Docked to a monitor / TV
The most important setting is to set the resolution both outside (Steam -> Game details) and inside the game (Visualization options -> Resolution)
Steam -> Game settings -> Properties... -> General -> Game resolution: 1920x1080
Scaling filter -> FSR (2)

In-Game
Y -> Misc. -> Video Settings

Visualization options:
Visualization mode: Fullscreen
Resolution: 1920x1080 (only available after setting the resolution in Steam game settings)

3D Graphics options:
Screen Brightness: 46% (set to your preference, your monitor may influence)
Smoothness quality: 4
Postprocessing quality: 1
Shadows quality: 2
Screen percentage: 100%
FPS: 60
VSYNC: Yes

---- Optional settings in Desktop mode ----
This changes the antialiasing algorithm to be a bit more efficient.

Open the Dolphin file manager and go to:

/home/deck/.steam/steam/steamapps/compatdata/1295510/pfx/drive_c/users/steamuser/My Documents/My Games/DRAGON QUEST XI S/Steam/Saved/Config/WindowsNoEditor/

Open the Engine.ini file with the editor and set the [/script/engine.renderersettings] to this:

[/script/engine.renderersettings]
r.DefaultFeature.AntiAliasing=2
r.PostProcessAAQuality=6
r.TemporalAASamples=16
r.TemporalAACurrentFrameWeight=0.45
r.TemporalAAFilterSize=0.5
r.Tonemapper.Sharpen=0.5

Reference: https://www.pcgamingwiki.com/wiki/Dragon_Quest_XI#Anti-aliasing_.28AA.29