First things's first, I'd hire an accountant, a lawyer, and an executive assistant, have them all keep each other in check and keep me financially and legally secure, and let me direct the use of that money more easily than having to spend it all myself. But that settled, I would fix the problems that personally affect me the most, first:
My mom is presently homeless and in terrible physical health, and her mom (my only living grandparent) likewise; grandma at least can stay with another of her kids for now, but I live in a tiny trailer and can't really help mom. But simultaneously, the first childhood home I can remember, near where I live now, is for sale. I'd buy that house in cash immediately and let mom and grandma live there, with in-home nursing care and a personal assistant to help them with anything they need, and instructions to help enroll them both in creative and social programs (painting classes, book clubs, etc) to help bring them both back into good enough mental shape that I can have positive relations with them again.
I'd buy my dad's underwater land off him, and basically redevelop the entire thing into a proper house, rather than the decaying frankenstein partially rebuild mobile home and unpermitted slapdash structures it's made of now. It would include a lot of garden space, so that dad could grow his own food, which he loves to do already, and which would save him a lot of money. I'd probably also buy him a lot of musical instruments, and let him retire to the music he's always wanted to do his entire life.
And lastly I'd buy the property at the top of the hill from where dad lives now, the ruins of the huge old house I grew up in as a kid, before the mountain fell on it. I'd have the whole place rebuilt the way I remember it, and retire there myself. I'd finally be able to live with my girlfriend, and we'd get married and live happily ever after in there.
Of course, new cars and computers and phones and such for all of the above, and physical and mental health care and therapy for everyone involved until everyone's in as good a shape as possible, but that's all trivial on the scale we're talking about.
I'd then invest enough money in some stable long-term investment like index funds, such that the returns are enough to fund the upkeep of all of the above and a comfortable lifestyle for everyone involved indefinitely. I would then spend my time completing the life's work that I've had to put off until retirement.
I'd probably finish the mod for an ancient video game that I never quite completed, and then make another mod for another ancient game that I never even got to start, just for a warm-up. I would likely hire a bunch of my old friends who wanted careers as video game developers to help me with this, and maybe fund a startup for them to bring their own projects to fruition together and realize their own life dreams.
Then I would finish writing my philosophy book (probably hiring a philosophy student or graduate from the local university to be my sounding-board and fact-checker, since I'm so out of practice in that world now). Then I would probably spend the rest of my life fleshing out the speculative fiction series I've been very slowly developing in one form or another my entire life. I'd probably also study a lot on my own, or take classes or hire teachers when I can't teach myself something from books.
I might want to make the personal-funding investment large enough that I could afford to travel while I do all of this, and then I'd buy a boat and sail from port to port along all the coasts (and up all the rivers) of the world, maybe traveling by horseback up the rivers where boats can't fit. Bringing an entourage with me the whole time to make sure we can communicate with the locals, get us through customs, stay out of trouble with the law, don't miss any important sights, etc. See the world, while doing my life's work, since all I need to write is a laptop and I can do that from any hotel room anywhere.
Any money left over would go to further investments granted to a foundation I would create to fund charities, and to create a charity. The charity I create would be a general-purpose help center, where people come first when they need any kind of help. It would direct them to other existing charities where they exists to help with those kinds of problems (and direct funding to those charities for the good work they do), and also offer custom help, including both guidance and as necessary (and affordable) funding, to solve problems not covered by existing charities. It would gather, in essence, emic ethnographic data from its clients, figuring out what it is that people in general are suffering the most from, and aside from publishing that information for the charitable economy (and private businesses who may see opportunities for new niches to serve), either boosting existing efforts to solve those problems, or founding new charitable organizations to address those problems specifically. Where possible, such new charities would operate by subsidizing the purchase of relevant services from private businesses, and then invest in those private businesses, using the proceeds from those investments to further fund subsidize the purchase of their services by those in need of them, creative a positive feedback loop that grows the charity's ability to give over time and helps keep it self-sustaining.