When a family office first holds Bitcoin in size, the question is never "which exchange?". It is "who can sign, who can co-sign, and what happens if any one of them disappears?"
The 2-of-3 baseline
Two hardware wallets on-site, one in an off-site safe with the family's legal counsel.
When to graduate to 3-of-5
Anything north of a single-digit-million USD position deserves a wider quorum.