Problem is that mainstream brokerages don't usually accept bitcoin or even list bitcoin as a trading pair.

Apart from 1broker, which arguably could be considered as a "crypto brokerage" as well since its customers are mainly bitcoin based, i really can't think of anything else. Obviously binary options will always have btc, but then binary options are more like gambling since the timeframe is usually very short.

I would say use either 1broker, bitfinex, or bitmex. All of them are legit and have been used. Of course there could be hacks and whatnot but that's the risk you are dealing with with every action you do on an exchange, not just shorting btc.