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Hello - fellow cancer mom here striving towards FIRE. First, I am so sorry you are going through this and send internet ((hugs)).
My son was diagnosed with an aggressive subtype of leukemia at age 13, bone marrow transplant age 14, now cancer free, 18 and graduating HS. We had started to plan to FIRE before his diagnosis, and will retire in a couple years - a few years later than hoped but still happening.
Sounds like you are still in frontline treatment, am I right? My suggestion is to wait until he is in maintenance to make any big moves. You will have a lot more clarity and know your son's needs going forward and are right to be concerned about the costs. My son has had about $3Million in medical expenses since diagnoses ($1M was transplant). Unfortunately, it is pretty likely your son will need something. Whatever health plan you get, you are going to want to make sure that mental health counseling is included in the plan as there will most likely be a need for it - possibly for your whole family.
An option to think about that may be available in your state is when your son is 18, he may qualify for Medicaid separate from your income. I've heard of parents doing this where their insurance is the first insurance, and Medicaid is the secondary insurer - meaning Medicaid pays your out of pocket deductible for your son.
We have a High Deductible Health Plan (HDHP) and a HSA, this ended up being cheaper than the next lower plan available at my husband's work if you are going to max out the deductible. The one nice thing about the cancer is you know you will max out your deductible each year, probably within a month or 2. Definitely look into a HSA if you haven't. One strategy we've used is that we piggyback the paying of our yearly deductible with opening a credit card with a sign up bonus that has a minimum spend of several thousand dollars in a few months. So we get the sign up bonus and then typically 1.5% or similar as cash back reward. Not a huge savings, but it does typically knock several hundred dollars off.
Also, document every medical expense paid out of pocket. Every time you pay to park, all your mileage to/from he hospital, etc. We went to another city for a second opinion and were able to deduct our hotel costs from the HSA. I'm not sure how far you can go back with an HSA, but if you ever do get an HSA, it is nice to have a stack of receipts you can reimburse yourself with any time - whether you need the money now or just sit on them while your HSA grows.
A final aspect to consider is you may not want to stop working in some capacity. You are supposed to grow apart gradually from your child as they grow and become more independent. Cancer throws that all out whack, especially with teens. They get sick, become very dependent like a baby or toddler, then get better and suddenly aren't dependent - it is not gradual. For me is was pretty disorienting as a parent, and one thing that really helped me get over our situation was going back to work. When I went back it was very renewing to not just be "My son's" mom and having something going on in my life that wasn't cancer and going to the hospital all the time. It has really helped me in developing a post cancer life again.
Hang in there and good luck.