Yes, I totally understand, your predicament. It is so hard on us, the owners, who love our cats so much. Your Dude is lucky to have you!

We've switched to Blue Buffalo Wilderness food now. My son recommended it. We were devoted IAMs Eukanuaba users.
The Wilderness is grain free, and 40% protein, and has antioxidants and cranberry (for urinary tract issues). Weezie who was putting on the weight, has lost some of it on the Wilderness. They all like it and eat with gusto!

Our two very elderly cats, Sheba (24) and Tippy (20) lived long lives on IAMs. They died a couple of months apart, last year. I was quite broken up over that loss.

We are trying out some grain free canned...3 types, but our cats are not liking it. So we will stay with Friskies special diet which only has rice in it for carbs. I give cooked whole meats to them anyway. So if they have to have the Friskies or 9 lives here and there, I guess that is where we will be.

Houdini is going to be neutered soon, and I worry about him and the male urinary tract problems that some get.

Keep an eye on Dude. When he appears to be in pain, I would have him put to sleep. We watch Oreo very carefully ...I won't let her suffer. I am so glad I didn't spring for the surgery. Her quality of life would have ended then. I think she has "lived" with gusto far more than most cats get to do as it is. My husband is very attached to her and she to him. She goes out with him all the time when he works outside doing chores. To see them together and her so happy, tail up and dancing around him, breaks my heart now. Don't know if she will make it til summer again. We have photo albums here in our profiles, and you are welcome to see Oreo and her joie de vivre! I have some cute pictures of her in the vacation section and family section.

Hubby is now building a close relationship with gentle, smart, Houdini. He was an outside feral, with a sweet disposition, and I adopted him thinking Oreo wouldn't be around very long. I hoped to have hubby transfer to Houdini, and it is now happening. Houdini is turning out to be exceptional too. We've never had a cat like HIM!

Dr. Mercola has a Vet series of YouTubes with Dr. Becker.
You might find her interesting:
Example:

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many videos on food, and other issues.
We are getting the new adjuvant free vaccines now, ourselves.