I have been using a Pebble Time Steel for years even after their acquisition by Garmin Someone corrected me it was FitBit not Garmin thanks to Rebble, a modded firmware that gives it new life, but I would like a smartwatch that can track my heart rate.
I don't mind using an older device especially if it means a cheaper price. A bonus for me would be if it still had some kind of community around it or a custom firmware like Pebble watches do.
I found an original Moto 360 with a functional battery for about $30 but I am not sure if due its age its functionality would be greatly limited.
Edit: It seems like Garmin and FitBit might be worth looking at. But I wouldn't know what devices to look at or if there have been any major improvements with recent revisions.
Edit: Just an update I bought a Garmin VivoActive 3 used and locally. It seems to have some solid build quality but appears to have significantly less apps and features compared to my Pebble. The heart rate sensor seems to work alright though. I am a dork for metrics so I feel like that should make up for it.
Heart rate sensors usually work best when the watch is worn tightly. Most people tend to wear their watch too loosely on their wrist, which lets in a lot of background light. Since these sensors are optics-based, that light translates to interference. Try wearing it one notch tighter than you usually do (or slightly higher on your wrist, if tightening isn't an option for you), and see if that makes a difference.
Also, for what it's worth, accuracy isn't as important as consistency. If one device consistently reads you at 120 BPM and another consistently reads you at 130 BPM during the same activities, you at least know that you're getting the same (albeit slightly scaled) results. As with most things in this space (quasi-medical equipment), most readouts are going to be an algorithmic estimate, as opposed to a true live reading.