Hi @erima ,
I know it’s been a while but here’s an update that might help you.
A way to access the underlying quimb instance of the circuit is by using the private class attribute _quimb_circuit.
You can see an example of how to access this attribute in Forum thread #8356.
Note: The _quimb_circuit attribute is private because it’s not compatible with PennyLane’s new device API, so things might break for certain workflows.
I hope this helps!