Really only interested in something like Graphene, Lineage, or a Linux mobile tablet that can work with a typical Linux distro and display over USB-C. It is just a casual conversational ask. Maybe one of y’all has tried or knows the answer. I won’t use anything that runs google stuff or is a pain to load a custom ROM.
Does it necessarily have to be an independent tablet when not connected to your laptop? If not, then look into a “portable monitor”. That should do what you’re looking for. USB-C on both ends. Plugs into your laptop and can be used as an external display / secondary screen.
The utility of having both would be enough for me to consider purchasing a tablet when I will not otherwise. It should only be a small amount of code in AOSP to work with any device. I am also half ass shining a spotlight on the ridiculousness of this not existing by default on all devices. There are even protocols for display over micro USB. At the very least this should be a F-Droid FOSS app. All devices should go as far as having this potential functionality like an EOL where the modem can be disconnected along with all other apps and only run the device as a video pass through so that no code vulnerabilities from abandoned orphaned kernels can run exposed to external sources. Then the user could safely select to continue using any Android device as a display.