I wish you every success with this. These things are not for the feint hearted. Getting step counting to work kind of finished me off for trying anything too ambitious that required lots of testing.
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
The reason for it being draft is the problems caused by sending a lot of data. There is currently no definitive cause found for this. I think it is not caused by the GPS code, but I can not prove that yet.
There are https://forum.espruino.com/conversations/384228/ and https://codeberg.org/Freeyourgadget/Gadgetbridge/issues/2996 for more info on possible causes.