If you are wondering whether Hanzi drawing is more satisfying, or more effective, on an Apple Watch or an iPad Air, the honest answer is that device size is not a minor preference here. It largely decides whether the practice works at all. And the satisfaction you are asking about follows directly from whether the practice works.
Why the Apple Watch does not work for this
A Chinese character is components arranged in a balanced square, often with several strokes packed into a small space. To form one properly, you need room: room for the parts, for their proportions, for your finger or pencil to move. An Apple Watch screen is simply too small for that. You cannot meaningfully produce a character with correct structure and proportion on a watch face; you would be cramming, not writing.
So as a writing surface, the watch is out. It can do peripheral things, a reminder to practise, a streak glance, but not the drawing itself.
Why an iPad is close to ideal
An iPad, Air or otherwise, gives you a large, comfortable canvas. There is room for the character at a generous size, for its proportions to be clear, and for controlled strokes, especially with an Apple Pencil, see writing Chinese gracefully with an Apple Pencil. The bigger screen also makes the practice clearer and easier on the eyes, the accessibility point from an iPad app with large, clear characters. For deliberate, proportion-aware writing practice, an iPad is about as good as it gets.
An iPhone sits in between: workable, especially for quick finger practice on the go, smaller than ideal but big enough to form characters genuinely.
On the dopamine question
You framed this partly as which gives better dopamine. The honest version: satisfaction follows real practice. Producing a character properly and seeing it confirmed is rewarding because you actually did it, the healthy reward we describe in dopamine-driven Hanzi learning. Cramming a character onto a watch is frustrating, the opposite of satisfying, because the practice itself is compromised. So the device that supports proper writing is also the one that feels good, the iPad, then the phone, not the watch.
There is no clever reward trick that makes a too-small screen satisfying. Good tools feel good because they let you do the thing well.
Where Hanzi Write Practice fits
Hanzi Write Practice is built for finger or stylus writing on a phone or iPad, where there is room to produce a character from memory on a real grid and check it, see blind drawing. An iPad gives the most comfortable canvas; an iPhone works well for practice on the move. A watch is the wrong tool for the drawing, and we would not pretend otherwise.
Pick the device that lets you write the character properly. That is the one that will also feel rewarding.
Join early access and practise on a screen with room to write.