What features are you looking for? What OS? What kind of cardio?

There is gadgetbridge for Android, where you can connect a lot of fitness gadgets to and all your data stays on your smartphone. (Check out supported devices to find which hardware to buy).

Then there is opentracks for outdoor cardio, that can either connect to heart tracking stuff directly (like the polar H10 for example) or get data from gadgetbridge.

In both cases you cut out the official app and thus the cloud, but can use commercial high quality hardware.

There are hundreds of open source fitness related projects and devices that work with them. So you have to be clear about your needs, wants and wishes.

Amazfit Active 2 paired with gadgetbridge.

Banglejs 2

Withings are french

You're probably not gonna find a commercial one that doesn't use AWS, GCP or Azure though

Edit: take a look at the bangle smart watch, and I've just now found out about gadgetbridge that you might want to take a look at too

i think garmin smartwatches keeps data offline. some of them dont even have wifi

So, you're fine with fitness trackers sending data to China, Russia, any company anywhere (because data leaks happen, or fuck it, why should any company have my data).

Or do you want a fitness tracker that simply respects privacy?