While searching for a Fitness tracker with alternative software and a good power life if found the Huami AmazFit.

It’s distributed by Xiaomi and have some smart watch abilities. (Notifications ..)

The Price for this watch is ~ 50 €.

At the beginning I used it with the official Xiaomi "Mi Fit" App. </s pan>

Therefore you need to create an account or use an existing one.

In addition to this I tried Notify & Fitness app, which have a better s tatistic showing in my opinion.

For getting rid of the Xiaomi’s app you can take Gadgetbridge as an alternative.

It is open source and compatible to some other devices.