Your best option is OTC trading and established tier 1 exchanges.
You must split the amount in small trades across multiples sites.
OTC:
Cumberland is ok, but we had an unprofessional experience with them and loss opportunity to cash out ether at $880.
I will go for OTC trading desks based in London. They are the most professional in the market and adhere to UK and EU rules.
Suggestion:
Bitfinex
Sell in $1M tranches $20-$30M using Bitfinex OTC program (They will only work with you if you are not residing in the US)
Bittrex
Sell in $1M tranches $20-$30M
Local Bitcoin
There are some MBS accredited traders that do $10M dollar trades. You can sell $10M through them
OTC desk:
sell $1 million per transaction to build trust with them.
MAKE SURE YOU HAVE THE THE REAL IDENTITY of the people you dealing with. (do video calls and record the video
)