Have always been a proponent of dollar cost averaging - monthly or quarterly, but am also a proponent of buying the drip in a separate account. As long as the dip has been below the last amount you bought at - by 10% say.
Cannot seem to find a model or explainer that points out the problem with this - if something was ok to buy at 100, why is it not ok to buy at 90 and sell at 96? If it never reaches 96, at least you lowered the cost of your investment a little bit.
As far as putting cash away - there are far more stable options that SPX to park cash.