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.

Not automatically. You had to click ‘sign up for threads’. Still, it was very seamless and even let you auto follow those you had on Instagram, so the user count was definitely inflated. It’s not surprising the number of active users are way down.