For those whose daily living expenses are the same every month, who have few family responsibilities and a stable regular strong income source, and who have a strong cash flow, automatic DCA may be a good option. But for those whose income is irregular and expenses are not regular, and who have to fulfill various family responsibilities, I think manual DCA is better.
This your ideology might seems logical on paper, but I want you to be understand that even when you have a stable income source or a stable discretionary income, your weekly or monthly expenses might not be stable because you can fall sick and may need money for treatment, or something may happen that may warrant you to spend money, so you can't be too sure about life because we live in a world full of uncertainty, that's why manually dca remains the best because will have to act pragmatically, base on what is at hand.
Let’s assume we were not investing in Bitcoin in the first place, probably you have other investment you may have put in your money. I guess you’ll still keep some funds as a cushion to that investment in the cases of emergency so you don’t end up taking money from your investment before the desired time and this emergency also include illness or an emergency expenditure, except you’re not keeping emergency funds in the first place.
Despite the emergency funds, I’m not really a fan of automated DCAing, I prefer manual DCA, because I won’t want to put my self through the stress of adjusting the automated/fix amount for DCAing whenever it seems my basic expenditure surpasses my weekly or monthly budget.