Been using some Garmins. Everything seems to work, although capabilities will depend on the functions of your watch. ECG only has regulatory approval in a few countries and mine isn't included, so I didn't bother getting a watch that supported it.
The nice thing about Garmin is they take a different approach from Apple and Google/Samsung - those put iOS and Android on their watches, which is why you only get 1-2 days of battery life. Garmins run a tiny microcontroller and low power displays so you get weeks of battery life, at the cost of a more limited range of apps (they have to fit in a few hundred KB).
Garmins are also supported to some degree in Gadgetbridge, although I couldn't say which metrics are computed on watch and which need a cloud connection - GB can't do those.