One thing to note: portable ECGs are <$100 on amazon. Not as feature rich as the apple watch, sure, but the watch only measures from one location, normally you want to measure from a few, usually at least 3: leg, arm, and chest.
the watch measures from two locations actually, through the wrist and through a finger on the crown. you're still right that 3 (and more) is better.
For each of the three locations I mentioned, you also need to contact the portable ECG with your finger. The finger is being used to create a closed loop. It is not a measurement site.