SPICE modelling output/input coupling in ultrasonic transducer driver
Here's an attempt at modeling (inductive) coupling for the base of Q1:



The oscillations seem to go on, which is no surprise. No idea what coupling you meant, or mean, it's some brute-force weak coupling with a low inductance (secondary). It could be capacitive, in which case you could try using the behavioural capacitor, but it's safer to just insert a capacitor from the load to the base (though not very "realistic").