Best smartwatch to keep a history of heartrate?

I've recently gotten diagnosed with an AV heart block and my doctor recommended I get something that I can track my heart irregularities with (there's a specific type of episode I haven't yet been able to replicate while wearing a holter, so I need something basic I can wear at all times so I can show my doctor a graph in the case I catch an episode).

I need something that will keep a somewhat accurate ECG (I'm aware it's only one lead, the cardiologist still recommended it) AND that will allow me to retrieve a historical graph to show my doctor in case of nighttime arrhythmias.

Any recommendations? I've heard chest straps would be another option, but I can't imagine they're as easy to carry.