My dad had cancer, so I definitely understand the fear. It's mainly a fear of the unknown, so maybe try making it more "known" for how things could play out, and see how comfortable you are with those options.
Here's a hypothetical... So you find out you have cancer sometime in the year 2030. You max out your deductible for that year getting treatment. Let's say that cost your whole $10,000 deductible, in addition to the premium cost. Now it's open enrollment for 2031. You buy a top of the line plan that covers everything, and it costs more per month, but less than your previous plan per month plus the $10,000 deductible. They can no longer ask health questions when you apply, so you now have a "Cadillac" type plan for 2031. You continue getting cancer treatment. One of three things will happen:
a) Treatment won't work and they will need to keep treating you, either with surgery, chemo, radiation, or some other fun thing they've invented to treat your form of cancer in the year 2030. You can continue to select your health insurance plan for the next year during open enrollment.
b) Treatment will work! You will go into remission. You'll need regular check-ups for the rest of your life. You may get cancer again, or you may not.
c) Treatment won't work, and you die in 2031 or a few years later, depending on the type and stage your cancer was in when it was caught.
I guess I'd think about plans for each of these situations, should it play out that way. You may want to get genetically tested for certain types of cancer; if you have a family history of it, maybe there are things you could pre-emptively.
The other thing is that, if you are younger than retirement age and you get cancer, you may qualify for social security disability benefits even if you aren't currently employed. It's not a ton of money, but it could help offset the higher cost of insurance or treatment.