> If my phone could act like an ultrabook/netbook when hooked up to a screen

You just described Librem 5.

The only problem is its hardware, which even a 5+ year old ultraportable laptop soundly beats and wouldn’t make for a very good desktop experience.

The StarLabs tablet would be much better suited here, but it’s also 12.5” which is so large that you may as well just get a laptop with more power and better battery life.