FWIW, IMO it is entirely reasonable to pay off the mortgage when you FIRE -- I had assumed you were earlier on in the journey, where that doesn't make as much sense. Any plan must reconcile two opposing considerations: maximizing your upside potential; and minimizing your downside risk. During the accumulation phase, the former is most important, because you have plentiful income and time to manage any bumps in the road. But once you achieve FIRE and quit, your priority shifts to the latter, because you have now determined that you have as much as you need (so the value of excess returns is minimal), and you no longer have a steady stream of cash coming in the door to mitigate rocky periods in the market.
So given that you are at "immediate FIRE" stage, I will give you two options.
1. As others have suggested, pay off the mortgage and throw the rest in VTSAX.
2. If 100% VTSAX feels too aggressive, set up a bond or CD ladder with some of the leftover money. Take 3-5 years of living expenses (considering inflation) from the remainder of the windfall, keep one year's expenses in cash, and buy individual bonds or CDs maturing each year of the following years (depending on how conservative you feel). Live on the cash for the first year, and then a year from now, you will have another year's expenses maturing from the bond or CD. Take that money to live on, and then sell enough stocks to buy another set of bonds or CDs maturing a year after your last current set -- you are basically replenishing the ladder.
The reason this works is that when the market crashes, you just don't replenish the ladder that year. Because you have guaranteed income for the next several years, you can just let your investments ride until the market recovers -- then you can sell and replenish the ladder for however many years you missed. And that security means that you can maintain the entire rest of your portfolio in VTSAX, because now market gyrations are irrelevant.