The discretionary income to be used depends on the level of your comfort and not even about the market environment and If you say buying on a Lump sum is worth while buying on the down side then to me you are describing buying the dip and not buying with the lump sum, because talking about buying with the lump sum has nothing to do with the market conditions but rather an investor decision to buy immediately with the available Lump sum amount irrespective of the market conditions,
Your observation is very good, it would be best to only buy during dips, but isn't it better to always buy gradually and leave a portion of that discretionary income for dips? Because the DCA method can have a routine or specific dates to buy some BTC, but discretionary income in addition to some additional income can be used for that, for the dip as well, then we would be buying intelligently without stopping our DCA plan.