My one piece of advice for your parents is to look into delaying social security until your father is 70. With your mother continuing to work part time and your father collecting a year of severance and likely to earn some income after leaving his company (by running the AirBnB venture, taking retail shifts, whatever), I'd guess their combined household income will still be in excess of $100,000 for the first year, and then $50,000 per year for several years thereafter, which frankly most people can live off of. They also have the $100,000 in liquid savings they can tap if they need to.
The key benefit to delaying social security is that the size of the monthly check grows quite nicely if you delay starting your benefits to the latest possible age -- I recall reading that the growth is around 8% annualized. In theory, this is meant to be value neutral and fair in an actuarial sense: since you start later, you draw fewer monthly payments before you die, so the increase in size of monthly payment is just made up for by having, on average, fewer monthly payments. But the reason this is good for your parents is that it is a very cheap and easy form of "longevity insurance" -- and given that your parents have a fairly small nest egg, there is a very real risk that they will outlive it if they draw meaningful amounts of money from it regularly. Better to spend some of it up front on living expenses when they are 66-69 and receive a boost to their life-long government-guaranteed monthly income stream, than to chance it on the markets with minimal safety buffer.
Implicit in your post is some question about whether you can or should provide money to support your parents, or change your employment or lifestyle habits to position yourself to do so. I would say the answer is a qualified "no," at least for the time being. Your parents are still capable of working and pulling in a living wage, and will hopefully continue to do so for several more years. They also have built up some net worth which can help cushion them and supplement their retirement. It sounds like you have a track record of success in building your own businesses and that's how you want to continue to earn a living -- don't make yourself preemptively unhappy by trading it all in for a position as a corporate drone.
The time may come, in 5 or 10 more years, where your parents may need more help -- either direct financial aid, or, even more likely, help managing their affairs and living situation as they get older and more frail. If they can work the social security system they may be able to swing a $3,000+ per month income (between delay credits for your father and what your mother will add), which is more than adequate for a healthy young or middle aged couple to live on if they are willing to be a bit spartan, but can be uncomfortably tight if they are too old or frail to do all their own housekeeping and cooking, have lots of health expenses not covered by Medicare, or similar things come up. At that point you will have to re-assess. But at that point you will also probably be a millionaire (given you savings and earning habits). You can work things out at the time -- they might move in near or at your home (depending on your setup), they might have to move to a nursing home, when they move they can probably unlock some of their home equity for more spending money, you might start paying for some of their expenses (fair is fair -- after all, they probably spent 18-21 years paying yours). Given that you will live a productive life and build your own nest egg in the way that suits you best, I don't see any reasons to change your plans for the foreseeable future.
You also did not mention any siblings. If you have any brothers or sisters it would be good to sit down with them (and, at some point, with your parents as well), to discuss their financial planning for retirement.
