The closest I've found so far seems to be something like this random brand:Yo...
So I took a look and it looks like there are only a few watches that offer ECG- the Samsung GW3/4, Apple Watch whatever, and Fitbit Sense and Charge 5.
It looks like they use spot checks rather than continuous monitoring, unless I am mistaken. It may be worth looking into if that's your major use for it.
I have plusses and minuses in my mind for all of these watches. I loved my samsung until they started inserting ads everywhere, and I hated wearing a 46mm stainless steel watch to bed or while riding. They also killed the watch for me by locking it to samsung watches only in the GW3 and 4 generations.The apple watch's battery won't last overnight otherwise I would have one. I have no idea why they made something that can track sleep but doesn't last long enough to do so. I have no experience with the fitbit sense, but I am currently using a Fitbit luxe to track sleep and recovery. I really like how small it is and how useful it is as well. Time will tell if it lasts, but if you're looking for ecg in a small form factor, the charge 5 may not be a bad way to go. I wear my luxe on my right wrist and my real watch on my left, and I leave the fitbit on for rides, swimming, everything.
I'm not sure when I last updated this post, but I used the galaxy watch for about 3 years, a vivosmart 4 for 1 year (before it died) and now the luxe for a little less than a year.
ECG Fitness Tracker Heart Rate Activity Tracker | MorePro
MorePro ECG fitness tracker V19 can automatic monitor your Heart Rate, Blood pressure, HRV,Blood Ox-yg-en, Sleep Apnea etc.
www.more-pro.com
HR, SpO2, HRV, ECG, sleep monitoring - the only thing it doesn't have is temperature monitoring...