Only one I recall (smartwatch open source)

https://www.pine64.org/pinetime/

Does this fit the bill for you?

I'm pretty sure Garmin has a good privacy policy - mozilla seemed to like them - and I got a fenix6 sapphire on ebay used for about $250 and I love it. Way better than my apple watch, particularly because I care more about fitness tracking and hate having texts show up on my watch.

Garmin Fenix! The new ones are very expensive ($1000), but I picked up a used Fenix 5X a few years ago when it was the most recent model for $350 and I love it! Battery charge lasts about 3 weeks on mine and 6 weeks on the newest ones. It has sports tracking activities for just about anything you'd ever do, a great phone app interface, doesn't need to be charged for weeks at a time, and has GPS topo maps that you can use while backpacking, cycling , jogging, or even driving. I was able to track how far I paddled in my kayak thanks to the GPS. Anyways, it's a great watch if you're looking for a fitness tracker as well as a smart watch to give you phone notifications, music controls, and that sort of thing. The only drawback is that the screen resolution isn't great on the older ones since they use eInk screens instead of fancy screens. The new one uses OLED, so it's supposed to look better. Oh, and they're not touch screens. That was a little clunky for me when I first got it, but it doesn't bother me now.

Maybe a watch supported by asteroid os is something.

Some models have support for heart monitor. Don't know if there is a fitness app or something like that.

Techlore did a video on fitness tracking a couple years ago. I don’t know if it’s still up to date, but it’s a place to start your search.

I use a Xiaomi Smart Band 7 and pair it with Gadgetbridge, and it works fine for my purposes, which is HR-monitoring during the day, sleep and workout sessions. I rarely interface with the watch itself (which is by design), so if you want more functionality out of your watch, then this might be a little on the light side feature wise. I tend to keep Bluetooth off, so I connect to it maybe once a day to sync data with Gadgetbridge, which I again export for analysis. A bit clunky to connect - I have to search for it first in the Gadgetbridge app, and only when it has found it can I attempt to reconnect. Maybe this is easily fixable, but I have not bothered to do it because I only sync once a day.

You do need to obtain a key first though, which requires a login to the Xiaomi servers. I used a throwaway e-mail for the registration. Gadgetbridge has no access to the internet.

I've been considering the Bangle.js 2, and while I don't think the heart rate monitor is really good yet, there is active development and discussion about how to make it better. If you're interested in helping out, this could be a fun project!

I like my Garmin gps watch, though they have gotten what i would consider too expensive...

I like my Garmin but keep in mind their phone app is a webview, this means all of your data is on their servers. Whether they have an official usage policy you like or not, things can change, servers get hacked, etc. It's possible to use it without the mobile app, but it's a pain.

I'm very happy with my Garmin Epix (gen 2). It's got a nice AMOLED screen, which works better for me cause I'm honestly mostly using it indoors, and I don't really need 50 days battery life or anything like that. 13 days on pretty high settings is plenty for me...

The watch has its own built in GPS antennas, and works fine without a phone during activities, since I saw you should about GPS stuff elsewhere in the comments. I'm pretty sure it needs to sync data to a phone eventually though, for full functionally.

This is probably overkill for you, and you can get one of many much cheaper options from Garmin since all you care about is heart monitoring.

Apple watches are the only ones I know that do the full EKG thing if that's something you're interested in, as far as other options. If you already have an iPhone, I've only heard good things about the watches too. Pretty sure their adventure watch only really loses to Garmin on battery life.

I had a look into this recently and the verdict was there's nothing really that good out there sadly.

I'm not 100% but i think Garmin has a pretty good privacy policy - mozilla seemed to like them - and I got a fenix6 sapphire on ebay used for about $250 and I love it. Way better than my apple watch, particularly because I care more about fitness tracking and hate having texts show up on my watch.

I just bought an xiaomi smart band 7 pro. I really like it for the price!