I have up to 9 days to think about it, since Coinbase won't let me withdraw until the payment settles, which they said up to 9 days.
Are there any advantages to keeping bitcoin in Coinbase? Or is it more advantageous to move into a private wallet controlled only by myself (such as Samouri Wallet)?
Another concern I have is how fees would work and when is the optimal time to withdraw to a private wallet.
No one exchange, even the most reputable one, can guarantee 100% safety to your assets. Everything may happen, starting from hack attack and finishing banal technical error. We know lots of examples of it about even the really trustworthy and famous exchanges. Of course owners of such exchanges value their high reputation and its users and compansate lost assets in most cases, but I do not think that such a risk is worth of it. For trading such amounts that you will not regret to loose are possible for keeping on exchanges why not, but not for a long periods of time. And huge amounts it is better to keep on a wallet with your private keys, seeds - it is a real guarantee for you and your assets. But not forget about important sinple rules of using it and be always vigilant.