At this moment, on Coinbase, you can instantly sell approximately 1000 BTC for $9.91M - about 4% under the mid market price against what is on the book. Assuming you are a US person (since you want to move the money to a US bank) and have no problem complying with their KYC policies, you should be able to get the money a day or two if you chose to accept a wire instead of an ACH. Just for comparison, for ACH in particular since you mention it, most traditional trading institutions (etrade, schwab, ameritrade) limit your ACH transfers to 250k/day - coinbase does as well last I checked. So you'd get your money about 10x as fast as if you tried to do it via ACH. Wires are typically unlimited.

Snapshot of live book depth at time of posting: https://imgur.com/a/rhi7ZKL

Taken from here, look for "view exchange": https://pro.coinbase.com/trade/BTC-USD

Coinbase has a $10k per day withdrawal limit. You have to contact them to get more. They don't specify how difficult or time consuming that process is.

It seriously isn't that difficult to get this limit increased.

Speaking as someone who watched a buddy go through this process it took him ~2 days to get his limit increased to the millions for a wire transfer.

In other words, it is not possible.

OTC desks are different than exchanges, and you won't be able to see their books. There are also trading algos meant to push things through the market without being noticed. Of course it comes at a cost, you'll usually pay a discount (up to 5% or so ) to unload a large amount of crypto.

Crypto is an unregulated wild west, so these OTC desks typically don't have BD licenses, might do prop trading and be the counterparty to your transaction and take your position on their books (and then try to unload it themselves at a profit), or might match trades with their own customer lists. It's typically a pretty low-tech operation running on chat rooms.

I've worked with a few of those firms.

Market selling that much Bitcoin in one place isn't quite possible right now, but you could do this in a day on Coinbase and get real dollars in your bank account. If you split it up and sold some on other exchanges like Kraken you could reduce the time required to sell it. You could also trade the BTC for other popular assets like ETH on exchanges without fiat markets then sell that off on exchanges that do.

CMC reports around $37B in global BTC volume right now, but the real figure is probably $2.5 - 3B, and of that it's fragmented in different fiat currencies with different restrictions on where those exchanges are able to send money. So while it's certainly not nothing and it's better than years ago, it's like a quarter of the average daily volume of shares in a single company like Apple.

google bitcoin otc like gemini, even coinbase pro can handle millions in a day. market is pretty large. have you tried?