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.