Sometimes people intend to put their self into difficulty by trying to use money meant personal needs to invest into Bitcoin not knowing that Bitcoin investment is a longer time money for Bitcoin investment should be money you won't be needing for long just like you have said that it should come from your discretionary income.
Perhaps I usually refer people to one of JayJuanGee's works ( JJG’s Outline of Bitcoin Investment Ideas) and I recommend using his strategies of self examination before investing in bitcoin, its paramount that any one investing in bitcoin should first look in to what JayJuanGee calls
9 principle individual factors, if one properly consider this 9 individual factor he or she wouldn't find themselves struggling with his investment as it has solved a lot of problems that might occur in future. unforseen circumstances cannot be avoided hence one needs to prepare for them but properly examining his or her incoming using the said strategy.
Also waiting for the dip will only delay your Bitcoin investment journey because sometimes it may not even dip investor who can choose to wait for the dip before accumulating Bitcoin are properly those that has gotten to their Bitcoin accumulation target or those that has been accumulating Bitcoin for the past 8 years or more may decide to little by little during the dip but for new investors is a wrong idea to wait for the dip that's why the DCA strategy is there to help an investor accumulate Bitcoin gradually regardless of the price and continue hodling, the dip should only be an opportunity for an investor to accumulate enough Bitcoin and HODL.
You're right! waiting is not always a good idea even in other investment especially bitcoin because the expect entry price may not come hence one is liable to miss out for this single reason. and this one of the reason why DCA method is widely encouraged to prevent the complaints that came from people who sticked to buying the dips as some came in late because they where hoping for a retracement in bitcoin price while the market was full in a positive move and hence they couldn't enter as they expect. Perhaps I do not discourage any investment strategies but it's best to remain consistent with an amount you can afford to do without for a long period than go all in and struggle with selling which is never an option.