Agreed with what everyone else has responded to counter the claim that a one-man family office can reproduce Vanguard's VTSAX. Such a family office also has to contend with other giants trying to claim the G.O.A.T. title for asset management companies [1] with incredible scale efficiencies. The stakes are "for all the marbles of the game" high [2], and they're using all the benefits of capital scale they can scrounge.

The likelihood of a single person delivering index tracking performance better than these giants is pretty slim. Better to drop the tranche designated for broad market passive indexing into one of these giants' funds as an institutional holder, then have the one-man office project-manage leading education of younger generations, help the family leader clarify the ongoing family mission the office supports, the accountants' filing tax compliance at the individual and trust/foundation levels (likely in many different jurisdictions), grooming a successor, updating processes and procedures to improve auditability/accountability/anti-fraud detection, assisting with legal compliance, helping out with financing approved moonshots, etc.

[1] https://riabiz.com/a/2018/12/13/vanguards-asset-machine-wobb...

[2] https://www.bloomberg.com/markets/fixed-income