Being married for 50 years is quite a wonderful and rare thing these days. That lifelong partnership takes on a new meaning when one of the members becomes critically ill. My husband and I will be married 48 years this next May and know first hand what you’re feeling with your husband’s illness.
We never know how strong we are until it’s the only option. I know it’s a trite expression, but taking things one day at a time is really the best approach. It allows us to stay positive for manageable amounts of time. It takes practice but it’s doable.
From what you’ve mentioned, I feel pretty confident that any of the 3 doctors you’ve consulted, considering that they are on staff at centers of excellence, would be very knowledgeable to treat your husband’s condition. At some point you have choose one doctor to get continuity of care and let them do the worrying so that you can reduce the stress over the illness and treatment. And considering your husband’s other health risks, making the journey to any of the 3 Mayo campuses might be more stress and physical burden than he wants to take on at this point. But if you are interested, here is a like to get the ball rolling. http://mayocl.in/1mtmR63
A referral isn’t necessary but it can be helpful to have a physician send in a referral along with patient records. But it all begins with a phone call to one of the 3 campuses on this link.

In all honesty, trying to get an appointment at a Mayo Clinic when your husband is already being seen at centers of excellence might be fruitless. Often when triaging appointments some of the factors they will use at Mayo would be redundancy of care. If you’re already getting the best care for a current health issue at a local facility the request for an appointment might be denied. So I would stick with the bird in the hand. You have great local doctors where you are. I don’t know what answer you’re looking for but when you have 3 doctors pretty much giving the same opinions on your husband’s condition, I’d pick the doctor that you feel you can work with and defer to his/her decisions. Waffling is just creating unnecessary stress for you.

If the doctor you trust the most feels it’s ok to watch and wait, then that’s based on results of the bloodwork, PET scans, statistics from years of treating this illness, and their firsthand knowledge of the disease.
Then do your best to relax and both of you enjoy life to its fullest. Try not to dwell on what the future might hold, appreciate the life you have now. Living with “what ifs” robs everyone of quality of life. I know your husband is exhausted and I’m sure you are too, with all the stress and worry. But try the Spoon Theory from my previous posting. It’s a great way to cope with the changes brought on by fatigue.

Worrying about getting Covid should be minor compared to controlling the advancement of the cancer if treatment is warranted. Avoiding the Covid virus is relatively easy and we shouldn’t live in fear just because our immune system isn’t up to full potential. I’m immunocompromised and don’t miss a beat out in the real world. I’m educated in how to avoid the disease and take precautions. I wear a mask always out in public without a blink of my eye. I earned the right to wear this mask and didn’t come this far in my journey to be defeated by a virus I can avoid. It’s an attitude your husband will have to take as well.

Sometimes as a caregiver, we get so wrapped up in concern.
I have to ask, is your husband as worried about all of this as you are?