Is there any simple rule of thumb as to when to trade against USD and BTC?
Maybe always trade against USD unless BTC is in Bull Mode /?
Be alert and wait for the value of BTC to fall from 5% - 7%, which is an opportunity for you to spend 33.3% to buy Bitcoin. After that, if it decreases by another 1.5%, you will continue to use 33.3% more to buy more and continue with DCA.
and when you buy all 1000 $, place a stoploss about 2%. After that, you only need to wait 2-3 days, the market will recover by itself and you can have 3-4% in a short time.