300 million? Hooo boy, I would -
1.) Get rid of all my debts.
2.) Buy a comfortable place to live so I wouldn't have to worry about it, with enough space for the basic necessities (bed, kitchen, bathroom, etc.), and a well-lit studio - and a library. And a small tower, if I'm feeling quirky.
3.) Spend aaaaaaaallllll of my time working on my own projects, learning and trying new things, without having to worry about bills or pensions.
4.) Do my best to support other artists via Kickstarters, Patreon-campaigns, setting up some sort of grant for creative people, etc., etc.