Jo, have you tried desensitising Bill?
If you have a smart TV with USB then download a fireworks display from YouTube. Outside of fireworks season play it at random times with the sound muted and just go about your business as normal. Slowly, over days and weeks, turn the sound up bit by bit and be sure to give Bill a treat when he doesn't react to a bang or pop or crackle, and soothe him if he does.
Sis lives in a quiet dormitory village twix an Olde Worlde coaching inn and a community centre and both have displays on the weekend nearest 5th Nov taking it in turns to do Friday or Saturday. Chester, their black Labrador, was in pieces and they tried all the remedies you mentioned, and more, till M hit on the above idea. Ches was never truly comfortable but it did at least help him to function rather than being wrecked.