Have you considered raw? I do realize its not for everyone but it does have the advantage of being totally free on any kind of additives so it does make pinpointing any allergies easier. Giardia thrive on carbohydrates so a diet low in carbs can help if you do have repeat infections.
I changed my dog to raw at around 5 months and we‘ve never looked back, have had no more Giardia and no more worms. I have his pooh tested every now and again as Humphrey’s a bit of a scavenger but I don‘t like deworming unless necessary, but its always negative.