I’d probably start with a trusts and estates attorney to deal with tax/security issues.
If I had $300m I’d put like $50 million into Vanguard or maybe even a robo investor and use just those gains & dividends for my daily expenses (general rule is it should reliably get you about $2 million/year in perpetuity).
The rest I’d split between a charitable trust to donate to crap, trusts for the kids, and probably the biggest bucket (maybe $100m-$150m) for potential big expenses or business ventures. I’d have someone help me with that last bucket but probably not the others outside of setting it up.