Long time user and donor, had GrapheneOS on my many pixels! I upgraded to a P9P XL and first thing i tried was desktop mode. It is not perfect and still has some issues but i can finally just plug in a usb c hub with hdmi port and there you have a desktop.
There are 2 ways of running it:
1) the classic and still buggy desktop mode: enable desktop mode and resizable windows under developer options. This mode can work perfect for 1 day or 1 minute, but it will just stop working randomly for me . it works better if resizable windows option is off but not reliable at all. the advantage of this mode is that you can use your phone to do your things and the display is not mirrored, it s another display entirely.
2) the cheap and dirty way: change smallest width settings under development options. this mode for me has been extremely stable. your phone display will become very hard to read since all the texts and stuff will be extra small but on the mirrored display everything will be fine. the display is mirrored and what you see on your phone is what s on the tv/monitor.
Is it ready to replace your desktop ? no . is it useful on the road if you have a bluetooth keyboard and a usb c hub ? yes