DCAing don't really matter if the market is high or low, so you don't have to wait till the market value goes down before investing. I agree with you about having an additional income or having a major source of income if you want to apply the DCA method, however making one big investment too is another strategy which is mostly the long sum method, which for me I believe is for people that have the purchasing powers to do so, however the whole idea is to best use a strategy that suits your income.
Just make sure that when you DCA, you're not spending the money that you're going to use for your food or utilities.
That's the reason why it is important to consider as well on what kind of money you'll use as you DCA. But some investors goes to the tough path and they're spending money that's allotted into something important but they are sacrificing that expenses to buy Bitcoin.
But don't do that, it's why we're all advising everyone to have some other source of income and only invest what you can afford to lose.