Any device that works with gadgetbridge

https://foundation.mozilla.org/en/privacynotincluded/categories/wearables/

If you're picking from the popular ones, Garmin is usually recommended even with their cloud services enabled. I know some models allow you to stop syncing data indefinitely. I assume it should be possible to only sync through gadgetbridge or the cable, but I haven't checked extensively. For example, if you just set it up without connecting it to Garmin services

If you make a list of what specific features you need/want/avoid, it might help narrow things down

OP, what your asking for might be a test of frustration. You'll either end up with poor hardware or lacklustre software.

But I guess it depends on your goal. For years, I used a pebble smartwatch via gadgetbridge for a fully offline experience, but it was so limiting.

I ended up with a Garmin instinct solar and couldn't be happier.

The Mozilla foundation gives Garmin top marks for privacy and transparency, saying:

Garmin has been on our Best Of list for a number of years. And we're pleased to say, in 2023, it seems they've gotten even better (which is amazing when so many other companies are getting worse) SOURCE

I do use a Garmin bike computer, too, so I'm acutely aware that my ride location data is being sent to Garmin. But for my use case, I'm OK with that.

I’ll echo what @otter@lemmy.ca said, Garmin is really the only way to go. Currently. You can basically run them completely disconnected from Garmin after you get them setup.

I’ll also echo what @ExtremeDullard@lemmy.sdf.org said but qualify it by saying the PineTime is…a piece of junk. I wanted SO much to like it, but nothing about it works the way that is intended. It will not monitor your heart rate accurately. At least not for longer than 30 seconds. It will also not keep accurate count of your steps. It just doesn’t have sensitive/effective enough hardware for it. The firmware and OS are barely maintained. It’s sad, but true. I’ll be the first person in line to drop $500 on an Apple Watch or Samsung Watch equal FOSS watch. The market just doesn’t seem to be there.

You're never going to get any sort of privacy with any device that runs WearOS because Google, and it can't be rooted.

Your best bet is an open-source smartwatch. I looked at true privacy-friendly, offline, open-source smartwatch options a few weeks ago, and my conclusion is that the only two viable alternatives are a PineTime or an older smartwatch running AsteroidOS, and none of the watches supported by the latter are sold anymore. So that leaves the PineTime.

Also, I wasn't looking for a fitness tracker myself, but the one thing that stands out with applications for non-wearOS smartwatches is that they're lackluster, to put it mildly. Fitness tracking with the PineTime is basically step counting and heartbeat monitoring. That might not be enough for you.

I see Garmins are being recommended but if you are looking for something cheaper, I've been extremely happy with my Amazfit watch. It has pretty much all the same functions as the Garmins but at a fraction of the cost, and is fully compatible with Gadgetbridge, so all your data is stored locally.

I have the Garmin instinct 2 and you don't need to pair it to a phone. Pairing it only provides a backup and a few pieces of information you can't get without the pairing. That way the data stays completely on the watch.