Garmin watches with Gadgetbridge, and if you're on GrapheneOS you can even deny it network privileges.

I hate to be a Debbie Downer here, but my advice would be to not use anything to track your sleep. I used an app to track my sleep for about 12 years, and the number of times I thought "I feel great today!" - but the app said my sleep was terrible (and vice versa) was so high that I gave up on it altogether.

I've also been reading more and more reports of other people seeing similar results with Oura rings, Apple watches, etc lately. Seems like it's all just junk science.

First of all, in general the sleep tracking is just an indicator. If you want to track getting enough sleep (just the hours) you're good to go. As for sleep quality: I completely ignore that part. It's rarely consistent with how I experience it and feel fit for training..

That being said, even though garmin seems pretty close to being the standard for runners, I went for polar. Mostly because of the data collection policy but 'buy european' put in a lot of weight too.

A lot of watches work with Gadgetbridge