Apple Watch Ultra 3

Only if we can get that data exported in a format that can be used by the next 'standard'. I've already got health data on my Android phone, a Fitbit account, my Withings devices, my old iPhone, and probably more elsewhere.

You can export all the data from Apple Health from a button in the app. It exports a zip with data in a usable XML format. The Health data is also available via an iOS API so there are apps available which pull and export the data in other formats (e.g. CSV). I try to connect any health related apps to Apple Health (e.g. Withings, MyFitnessPal) and use that are the central repository.

On android I'm able to extract some of this data by directly accessing the sqlite files in the databases directory of my rooted device, (with sudo-level permission obviously). I find that much more comfortable than having to use APIs with temporary tokens

I would love to try it but I'm guessing it won't work if I just buy a cheap used one and never attach it to an account and never bring it online?

Opposite direction, but I was able to export most of my activity like sleep and daily steps from Fitbit to Apple Health (using a 3rd party app unfortunately.)

Not sure what you mean by "one" but a watch will work fine without a network connection. It does need to be paired to a phone.

It'd make it less useful as a watch though because the time will drift without an NTP server.

I don't think this is true? All my Apple Watches refuse to provide any functionality at all, including just telling the time, until I pair them with an iPhone. This was an issue when I bought my first Apple Watch because I owned an Android.