Might be best to avoid smart watches, they are truly little SPY devices…

If you use an Android phone, you can install Trail Sense from F-droid on it. It has a step counter. Then if you still want to wear a watch, use an ordinary dumb watch.

https://f-droid.org/en/packages/com.kylecorry.trail_sense

I have the old CMF Watch Pro, which is compatible with Gadgetbridge, which is a FOSS app, and supports all features. The new ones are also compatible but may lack a feature or two. You can use it without connecting to the phone too, but I don't mind, since Gadgetbridge is FOSS and privacy friendly.

For the battery, mine works for at least 3 days, when connected to the phone and using other features like always on display, lift to wake etc.

Have you looked into asteroidOS?

PineTime is probably the best choice privacy wise. It hits all your requirements but it is definitely not fancy. Pretty much everything will require an app, but the GadgetBridge app is FOSS with no trackers or uploading your data.

This is just my mindset so take it with a grain of salt.

I think for outdoors, fitness, and sports you don't get a Garmin you shouldn't get a smart watch dedicated for it. I personally run, hike, climb, occasionally even dive. I just wear a simple casio square day to day because I don't want to shell out the couple hundred dollars for a garmin fenix just yet.

I don't particularly know what's your use case that some burner phone that only has a step counter app can't do

Well https://www.amazon.com/Casio-A158WA-1-Water-Resistant-Digital/dp/B000GAYQJ0

Don't buy it on Amazon btw.