Hi Lori!
I was reading your condo from yesterday - did the vet explain to shoot the Simbadol onto his gums for best absorption? With my kitty Pearl who has chronic pancreatitis, I don't even try to open her mouth, I just slide the syringe in at the corner of her lips and squirt it.
Buprenorphine can really hit some cats hard. What is his dosage? If you feel it's too much (he is too lethargic and out of it), you could always try squirting in 1/2 of the amount. My cat Whisper had a sensitivity to Bupe and it didn't work well for her, it affected her breathing (she is the black cat in my profile pic, Pearl is the Siamese).
I know you are doing the Cerenia (at least I think I remember rightly), so bravo that is another big piece of managing pancreatitis. Sub q too. Even when Pearl is not flaring, I do the sub q 1-2 times weekly to help with her hydration (but I know you have to be careful about that with the murmur - the cardio appointment should be able to confirm for you if/how much fluids you can give regularly). She tends to have very hard bowel movements also without it.
Also, when Pearl is having a Panc flare, her BG increases. Marje helped me understand when first diagnosed with FD that I need to increase the dose when needed (even if I suspect it's in the middle of a pancreatitis flare and the pancreatitis is the cause of the extra-high BGs). That could really be affecting your boy, and I hope as he starts to feel better you will see it in his BG readings, too.
I also want to add - Pearl is also an asthmatic. You can definitely manage asthma and FD together. Inhaled steroids really should not affect the BGs. They go directly to the lungs and target only the lungs. Steroids that affect BGs are oral, most common Prednisolone. Have you tried the inhaler steroids? Asthma attacks also increase and keep BGs elevated, I have found with Pearl after she has an attack or is struggling more with her breathing, it's definitely reflected in her BG.
Hang in there! Seems like Max and Pearl have some things in common!! Hugs from Pearl to Max!