I suppose that perfect posture of the photo will disappear after a few minutes of use.

Maybe there exists some kind of selfie stick / tripod that allows you to place the phone at eye height?

This is actually a setup I might prefer over using a laptop.

It doesn't even look perfect - rounded shoulders, forward head, not much lumbar curve. Probably all inevitable when you have a tiny keyboard and screen is way too low.

Also, good posture in a chair is nearly impossible to have, let alone maintain. I've started working from the floor and sitting with my feet/legs in a variety of positions - immediate and immense improvement in my posture, overall mobility and reduction in pain.