The best advice is go to Bessemer Trust in New York and let them handle everything, they have been in business well over 100 years.
I would go to four separate Investment Banker / Trustees. I would have them monitored by a Trust Attorney paid by the year and after five years the Trustee with the poorest communication and security performance would be replaced after joint evaluation with the Trust Attorney. Each would be charged with diversification of investments so that no more that 2.5% of their money managed was in any one stock, RET or other vehicle.
That’s the way we did it on a pension fund I sat on the board for for five years.