Thanks for the response and linking to your code, @otso. One significant point of difference for me is that in your case the tag is deciding when to switch into the other operational mode, where as in my use case, this cannot be inferred and should be somehow prompted from a connected device.
One approach would be to connect and rapidly query a GATT characteristic, but from this thread I assume it’s not possible (without custom firmware) to trigger fresh sensor sampling via GATT queries?