Persistent BLE connection/bonding

Is there any way to "cache" the primaryService for later use (and store it in a JSON or something like that)?

There is, yes! Although normally you'd cache them in the app, not between apps.

I believe all you really need to do is do:

st_status = new BluetoothRemoteGATTCharacteristic();
st_status.handle = ...; // i think - it may be called something slightly different

... and get the handle from the number in your existing code. The handle number won't actually ever change on that device - it basically only changes with different firmwares/device types.