completely offline sensor that just did heart rate tracking
There's the Pinetime watch, which is pretty cheap and innacurate, but it is offline. It (and RTOS smart watches in general) are limited and scope but can serve your usecase.
There is also Asteroid OS, which is kinda dead but can I think track your heart rate without connecting to the internet or a phone.
The Pebble e-ink watches are now open source, and can function without a phone. I'm not sure if they have a heart rate monitor.
You can also strap a Samsung Galaxy S5 mini to your wrist and jailbreak it.
There are also full android smartwatches that can probably be customized to not connect to anything, but those are all bulky chinkslop. I stopped researching these after I learned that they are all bulky chinkslop so I don't know much about them.
Wear OS is the worst thing to happen to smartwatches. Unlike Android, it is closed source, which makes it way less flexible, limits what can be done with it, and makes something like Lineage OS impossible for smartwatches. Its been out for more than a decade, and yet the software available for it is extremely limited. Wear OS developers are so few and far between that even Apple's Watch OS has more (and better) apps on it. Wear OS watches lack customization, you are stuck with whatever launcher/UI Google/OEM gave to your watch. In addition, most of the functionality of the watch is locked behind Google/OEM apps, with no good alternatives.
Normally I wouldn't care about any of this, but I really want a smartwatch that can work like XMPlay but on my wrist (with a headphone jack). The closest thing that is being made to what I want is overpriced chink garbage with no support for a lot of the formats that I want to listen to.