Depends on a lot of factors, but assuming I had a fairly decent buffer of money saved up (which in reality I do not), my number one priority would be to do everything in my power to radically shift the variables in my life around so that I could make a move to a location in a warm (probably coastal) climate where the sun shines nearly 365 days a year, and where I'd have access to high quality food. I'd also prefer to have the ability to expose my entire body to the sun without being hassled, so I guess I'd have to find a nude beach nearby or figure out a way to do it at home.
This is all assuming that the cancer hadn't already progressed to an extent that it limited my physical ability to make these things happen on my own terms. Ideally I'd consider Mexico or somewhere in Central America, but since I don't speak Spanish, my options are likely limited to California or the southwestern USA. I haven't spent time in coastal Texas or Florida so I'm unsure if either of those places would make for a supportive environment or not.
I'd make it a point to obtain a large re-fillable tank of CO2 and whatever other miscellaneous equipment I'd need to rig up a system that would allow me to take frequent dry CO2 baths (in other words, encapsulating my entire body, except for my head, in a large bag of carbon dioxide for an hour or so at a time). Frequently soaking in a bath of magnesium chloride + baking soda is already pretty important for me to include in my lifestyle so I would continue doing that.
Not going to comment much on diet other than keeping PUFAs low, avoiding estrogenic foods and avoiding explicitly thyroid suppressive things like raw cabbage and other brassicas, soy and other legumes, etc. I do fairly well on good quality dairy and definitely require liver and oysters in my diet so those would all be necessary to have access to. Coffee, fruit, potatoes and honey as well, but those are all pretty easy to find anywhere.
Supplements that would be nice to have on hand would be aspirin, methylene blue, niacinamide, vitamin E, pregnenolone, DHEA, gelatin, and probably some others that I am forgetting.
That's what I would start with at least.