
The Kennel Club Assured Breeder Scheme isn't a guarantee of anything, except that the breeder has done 'some' of the health tests, which should include hips, elbows and the BVA eye certificate, and they are recommended to do the DNA test for PRA as well; that they look after their dogs and have a nice whelping area, which is only checked every so often, and they are then entitled to a discount off KC registration. Unfortunately, as you can see from that brief description, that allows all and sundry to join and do the bare minimum, a lot of breeders have left because of this, there are known puppy farmers on there all benefiting from discounted KC registration.
While there is no guarantee that pups won't go on to have problems with hips/elbows, good breeders would look at similar lines, check what stud dogs are producing, and what is behind the pedigree, and also now, use the estimated breeding value to see the probability of producing pups with problems. At the end of that it's still no guarantee, but at least the breeder has done all they can to try and prevent problems.
As for the stomach issues, I'm not sure as I don't have much experience, my girls (currently three Labradors, I lost my eldest age 14.5 earlier this year, a flat coated retriever and a foxhound) have cast iron stomachs, all are raw fed with a few home made stews thrown in, but if I run out of raw then they will eat anything else I get in for them pretty much and absolutely no problem. There are some sources that question if allergies 'could' be linked to annual vaccinations, I don't do this with my girls, they get their vaccinations as a puppy, then the first set when they're a year old and then that's it. I also don't use flea/tick prevention, I use natural products in their food, garlic, turmeric, fenugreek, mint and a few other bits. If you do continue to vaccinate then I'd suggest you follow the World Small Animal Veterinary Association's guidelines, and follow the three year protocol to avoid over vaccination. As for flea/tick prevention, a good product to use is neem oil, you can buy sprays containing this that deter parasites, and there's a product called CSJ Billy no Mates to deter all parasites, both much more natural rather than using strong chemicals regularly on your dog.
Yes, I would expect at least a level of moral support from the breeder, but then from the other side, puppy owners do ignore advice, which is also frustrating. One of my pups from my first litter was neutered under a year of age, against my recommendation as the vet said it would calm him down. And was also switched over to Royal Canin food again, on the recommendation of the vet. When we had a meet up he was the worst behaved with the worst dry coat of all my pups/dogs.
At the minute you would be hard pushed to find a good breeder with pups on the ground, I'm forever being nosy looking at litters, and have been shocked by some of the breeding going on to take advantage of the hugely inflated prices of pups currently. The latest one, a puppy from a bitch affected for PRA, and the dog untested, and looking back through the pedigree, that puppy could go on to go blind as he has the potential to be affected status. If you are going to look at adding a pup, then you need to get on a waiting list sooner rather than later, and do your research, make sure you like what the breeder is doing in terms of health tests, and ask them to explain why they're using that stud dog, what they hope to achieve with the pups etc. For me, they should be doing hips and elbows, hips should be around or below the breed mean standard of 9, elbows should be 0. Both parents should have a current clear BVA eye cert - all these schemes are now back up and running after lockdown. And at least one parent should be tested and clear status for PRA, CNM, EIC, HNPK and SD2, if one parent is clear then none of the pups can go on to be affected.