Fitness tracker or activity trackers are devices or applications for monitoring and tracking fitness-related metrics such as distance walked or run, calorie consumption, and in some cases heartbeat. It is a type of wearable computer.
As with anything that can be bought, I usually first try a cheap model to see if I need the advanced features that the expensive ones offer. After a quick model review, I went for the Amazfit band 5.
I've now discovered wasp-os an open source firmware for smart watches that are based on the nRF52 family of microcontrollers. Fully supported by gadgetbridge, Wasp-os features full heart rate monitoring and step counting support together with multiple clock faces, a stopwatch, an alarm clock, a countdown timer, a calculator and lots of other games and utilities. All of this, and still with access to the MicroPython REPL for interactive tweaking, development and testing.
Currently it support the following devices:
Pinetime seems to be a work in progress, Colmi P8 looks awesome, and the Senbono K9 looks good too but wasp-os is lacking touch screen support.
So if I had to choose now, I'd try the Colmi P8, the only thing that I'd miss is the possible voice assistant support. They say that you can take one for 18$ in aliexpress.