I really want to try NuVet. The dog's version contains alfalfa though, which our guy is allergic to. They told us to try the cat version, which is the same without the alfalfa, but it makes me wary, especially with the cost. Right now we have Free Form Snip Tip capsules (fatty acids) and Gentle Digest probiotics as rec'd by the vet. I loved the Prostera probiotic by IAMS that you must refrigerate, but they went up to $35 for about a 3 week supply so I had to stop those. I liked them a lot though and saw good results - not so sure yet about the Gentle Digest, it's only been a few weeks.
Good luck with your immune boosting! Sounds like you are on a great track!
PS Have you done an allergy panel? Our guy is allergic to a lot, including potato (white, not sweet), which the majority of grain free contain! It's been so hard to find a grain free good food for him, and have only found ONE that doesn't contain any of his allergens - Earthborn Meadowfeast.