I know keeping fiat on any exchange is not wise while waiting.
You should be fine with leaving fiat on exchanges because exchanges getting hacked is mostly concerned with stolen bitcoin and cryptocurrencies and not fiat; Assuming you're actually referring to real fiat money and not stablecoins.
In the past, exchanges have given 'haircuts' to all customer balances when crypto was stolen from the exchange.
This means if an exchange has $100m in total customer assets, $1m in funds in excess of customer assets, and has $11m stolen, customer balances would be reduced by 10%.
Someone wanting to buy coin on dips is going to have to give up some security in exchange for the potential additional profit. If you want to employ this strategy, you should do your own research and deposit fiat on an exchange you believe is financially stable enough to survive a modest loss of funds via a hack, and is overall unlikely to get hacked.