If it's dismissed on one device, it should also be dismissed on the other. Besides that, answering 5 calls shouldn't leave you with 5 "Incoming call" notifications, especially so if you answered them from the watch.

Apparently it does keep track of HR intervals

No, it's either measuring or not, it can't do intervals nor save the data. The gaps you're seeing is just data not getting synced because the screen needs to be active for it to maybe decide to sync at some point when it feels like it.

I think it’s reliable though.

The Osama Casio costs 23$, is also water resistant, the battery lasts ~7 years and can be easily replaced when it runs out, it has a working stopwatch, and a timer that can go for over 24h. Meanwhile with the pinetime you have to chose between risking it dying when you wash your hands, or throwing it away in a few years when the battery dies and you can't replace it. What reliability are you talking about?

I don’t know if there’s other smart watches that do more or cost less that are also open source and similarly usable?

Lilygo ones have a really crappy battery life but the models have a combination of WIFI, IR, LORA, GPS, and mic + speaker. So, they're much better as programmer toys, but even worse as watches.

Banglejs 2 costs about as much as the EU version, but the device is so much better it's not even funny. I'm pretty sure a lot of programmable Aliexpress watches are also running espruino, and it's got community ports for other watches.

TLDR

It's crap, and I'm still salty because the person ordering it for me (of course the EU store won't ship to non-EU European countries) got scammed without checking in with me whether there's something off about paying 3 times as much as what's shown in the link that I sent them...