My husband was diagnosed 15 years ago. He initially suffered from lack of saliva, as was expected post chem-radiation. The last several years he has experienced exactly what you describe--excessive saliva with drooling and spitting. It is also interfering with his speech clarity which very distressing to him. He has always been naturally outgoing and social. He is shortening his conversations and not engaging with new people as he once did. Swallowing is also deteriorating. Between the two issues, dining out even with friends, is becoming uncomfortable for him.

He mentioned this to ENT at annual check. Nothing was suggested for excessive saliva. Scheduled for endoscopy due to instances of food getting stuck with severe hiccups.
I wonder if late side effect of excessive saliva has not been studied because in past patients did not survive this many years post treatment.