I would recommend LedgerX -- A US based CFTC regulated derivatives clearing house and brokerage. They're available to US persons and organizations including retail. I believe non-US persons need to form a US company to use the platform currently.

The swaps and options on LedgerX are fully collateralized and physically delivered.

You can short Bitcoin there by depositing USD via wire transfer and buying puts.

You can see the current list of contracts: https://trade.ledgerx.com/live

The contract size is 0.01 BTC and prices are quoted per Bitcoin (similar to how equity options are usually 100 share contracts and quoted per share).

I've been a customer since 2017 and have been pretty happy with them. If you have questions, I'd be glad to answer them: I'm a seller of puts and would always welcome more people taking the opposite side on the platform. :)