Take up hand embroidery? Or make a homebrew app for Nintendo DS in ANSI C?

It sounds like you owe yourself some kind of passion project.

Good luck with the treatment! There's really no need to worry about things you've already done your best to sort. You've done your best and put yourself in a winning position.

Just focus on getting healthy again. You have a very challenging road ahead of you, and it's only about your health.

Money, job and responsibilities can wait.

Knowing the basic knowledge and how to bridge the gap will be very helpful in the next upcoming wave. You will have those who know all of C and those who know all of A but how you connect B?

If your confidence in Linux, switch it up. FreeBSD could do no harm, Haiku. Networking is another path to follow. Learn how to create a network, a home lab, install as AI Interface. Buy a VPS and start mucking around.

I'm a Systems Architect so I have to constantly morph in to new areas when they appear. It's a challenge but I have more "know-of knowledge" than proficient but it keeps me employed.

"What if I integrated X with Y" and see if it works are always fun experiments.

Try not worrying about the job. Phone it in for a bit. You have cancer no one will blame you and if they do you can say "Sorry, I have cancer, now what were you upset about again?" Don't let a corporation ruin your life, and definitely don't quit when you can't get fired.