I just wanted to share my experience with using Raspberry Pi 4 with display connected via usb/hdmi as a tablet/computer running QtAgOpenGPS, camera player etc. Since there are some Linux desktop environments developed for small screen devices (phones, I myself own one), I gave it a try and put it on Raspberry Pi device.
I use Manjaro ARM edition for Raspberry, where I installed Phosh environment, made some modifications/tweaks and now it is pretty much usable with QtAgOpenGPS for navigation, VLC for camera streaming or browsing internet via Firefox. Here is the video (note that I did not have camera connected - so the screen is black only - and the display has resistive touch, so the touch experience is not great sometimes):
Maybe it will be possible to run there AgOpenGPS some day, but that is another story..