Hello and welcome to the forum. Regarding the pancreatitis, was this a one off, acute attack and is your Schnauzer now recovered from it or has the vet diagnosed it as chronic? We have a dedicated thread on this which you might find helpful - link.
I am hopeful that you will find some useful information in it.
We have a thread on diabetes here.
Prescription food can be helpful in the short term but as you say, long term is a different matter. For both of these conditions you need to be searching for food containing high quality protein and not too much fat. This criteria is met in the fresh food product Butternut Box
and there are a few reviews from people who have found it helpful in stabilizing their dog's pancreatitis. This company has a helpline and may be able to advise further. With diabetes any dietary change needs to be discussed with the vet. He will need the dry matter figures for protein, fat and carbohydrate and will probably comment on the high protein but do not worry about that because it is highly digestible protein and from what you say about your dog losing weight, he might well do better on it.
If you want our help in searching for another type of food using the Dog Food Directory, please let us know.