JsVar leak ?

You're not unlocking the string created by jsvNewStringOfLength...

This should be fine?

void jswrap_ESP32_test(JsVar *text){
    char txt[20];
    jsvGetString(text,txt,sizeof(txt)); // this is a short replacment of the data I get in Bluetooth
    JsVar *tmp = jsvNewStringOfLength(sizeof(txt), txt);
    jsvUnLock(tmp);
}

I would strongly advise not keeping a global JsVar* around though - because you have to be sure to get rid of it in a _kill handler for your module (and also initialise it to 0!). If you're planning on storing data, I'd do it as a named JsVar in execInfo.hiddenRoot and grab it whenever you want it instead.