I have read dozens of threads saying you need to keep your BTC on a hardware wallet or paper. Anything online isn't safe they say. When trading that is not really feasible is it? I like to move in and out of positions several times a day and often at a moments notice. My trading account has a sizable balance (to me - 5 figures) in it.
How risky is this and any additional things I could do?
You need to decide how much of your total bitcoin stash you are comfortable losing, and only send that amount to the exchanges, and keep the rest in your own hardwallet.
Also take profits regularly and withdraw profits. That way, the % at risk is lower.
Sadly exchange risk is real - as those who had money in MtGox, Mintpal, Cryptsy, Bitfinex and BTC-E can attest.
The only group that is insured against hacking is Coinbase/GDAX. (Though Gemini might also be insured, I know they are fully licenced by New York State and I think the licence requires reserves to be held).