Looking at the list of supported watches, there are only 2 with 5 star support and they seem quite old / obscure. I couldn't find any to buy second hand anyway.

Nah, it is pretty much if you didn't buy one of 2 trendy models of the year, then nothing else has ever or will ever be supported (of course you can always write your own drivers but it is a ton of work, especially for non-coders)

I have a thought that a lot of the enthusiasts that go through the pain and effoet of writing all of these drivers for old phones they have were usually the kind of people to buy the best/most popular device of the year

we share the same superpower.

point it however, that is the nature of open source projects, just up to yourself willing to sacrifice time and hardware to test and integrate on your device.

This is all magic to me, but I getting contactleee payments on this and some form of music maps car projection from a linux phone and I would be all it.

I fully agree with you in wanting to keep your health telemetry private. I too baulked at smart watches for the same reasons. I hate the idea of such info being collected by any company.

I've found GadgetBridge paired with a cheap smartwatch to be a good solution for me. It let's you keep your telemetry local and provides a GUI for the info I want: steps, heart rate, activity, sleep, etc.

It sounds like they have some support for AsteroidOS already too. They also claim "As of the next Gadgetbridge release, it should have full support for AsteroidOS, and feature-parity with AsteroidOSSync."

If I'm not mistaken, some of the higher-end models from Polar and (maybe?) Garmin work independently of a smartphone.

Just stare unblinking at a clock. When it suddenly jumps forwards several hours subtract the difference. Easy.

Doesn't work when crossing time zones or during daylight savings transitions.

Not yet it seems. Also looks like it doesn't have a pedometer function. Hopefully a future version will add those, along with some way of downloading that data

Nah, it's not "too heavy" the linux kernel is running on all kinds of tiny processors.

My limited understanding is, that usually the problem is a lack of support and assistance from the hardware manufacturer to optimise the OS for their hardware.

This is why GrapheneOS and similar have poorer performance on phones et cetera.

lol no, these smart watches are powerful as older smartphones, their stock os is already android/linux based.

AsteroidOS mostly supports watches that come stock with Wear OS, which is a modified version of Android, which, guess what, runs a Linux kernel. These watches are on the more powerful end of the computing spectrum. As you say, there are a lot of smart watches that use pretty lean MCUs, but those aren't running AsteroidOS or Wear OS, as noted in this FAQ entry.

I mean, look at the specs of a smart TV or smart fridge. The specs on them are shit and they run Linux.

Hell there are whole distros designed to run on minimal as fuck hardware.