First things first: You own the company nothing. This is a professional relationship, nothing more. There's nothing wrong with quitting if the job isn't a good fit.
That said, if you quit I would not lie about your health as an excuse. Be honest about your concerns and they may be willing to modify the position to keep you, which is a win-win. If not, you were going to quit anyway so no problem.
Something else to consider: From your description it sounds like you're in a great position. This is a key role they desperately need and have had difficulty filling. In other words, this could be a great opportunity. Show them you can do the job and do it well, even if this means you're underpaid for a year or so. Then negotiate a promotion and comp increase. You could leverage this into a quick jump in net worth over the next few years, assuming of course this is interesting to you. If not, just FIRE and don't worry about it.
Thank you for the feedback. Yes, I think I get too involved sometimes. I like my immediate team, but the management parts of the job are not what I was expecting. I thought I would be more hands on but it is very high level cyber security.
You are right, and I shouldn't lie about my health, and I should have been more descriptive. At this point my health is stable but my numbers are certainly showing I am in a bad area. Now if my numbers stay like they are I could live another 40-50 years. But if they start to turn south, well then. I honestly don't know how things will go and if they do go south I don't want to regret not giving myself however much time I spend at the job with my family. As we have FU money, my partner will continue to work, and Japan provides a good safety net (and the weak yen for now is a bonus as FU money is USD) it seems like a no brainer.
I realize I am answering my own question here, but still, I have a sense of guilt over leaving the job for what I am considering a "selfish" reason.
Yep, out society conditions us to believe that doing things primarily for our own benefit is "selfish," which is batshit insane.
I would do some deep reflection on why on earth you are feeling its "selfish" to do the extremely normal thing of leaving a company.
ALL employees leave. It's a given, they all leave. Even the ones who die at their desks leave. It is literally the ONLY universal thing about each and every employee who has ever worked for anyone in any role. They leave.
Also, your job is someone's dream. You leaving allows someone else to fulfill their hopes and ambitions. The company doesn't need YOU, it needs someone who can do the job well. And yeah, whoever takes over isn't likely going to be as good as you, but you yourself likely weren't as good as you are now when you started.
Again, that's incredibly normal.
You're feeling guilty about the ridiculously normal life cycle of employees at a company.
Lastly, even if your guilt was valid, it's not a reason not to do something. I've felt legitimately guilty several times in my life for leaving partners who were perfectly lovely people, but I just wasn't getting what I needed from them.
Should I have stayed with my ex, lovely person, and gotten married and had a mediocre, but respectful marriage just to avoid feeling guilty about hurting them by breaking up with them??
Guilt should point you to examine a decision, but it doesn't dictate decisions. Breakups are the natural life cycle of courtship. Employees leaving is the natural life cycle of companies. Yes, harm is caused along the way, but that doesn't mean staying is the answer.