Instead of trying to buy a whole Bitcoin, a person adopts the DCA method which can be expensive, which allows us to buy small amounts every week or month or regularly, depending on our ability.
Expensive or not, it depends on price of thing you want to buy. DCA (Dollar Cost Averaging) is only a method that is free to use, has no cost of purchase, so it is not cheap or expensive.
This method allows us to invest quickly without waiting for financial stability, gradually builds our Bitcoin number over time and even helps us to average our purchase price, helping us reduce our risk.
You misunderstood one more point.
With DCA method, you can not invest quickly, and by saying it is quickly you were wrong. You have to invest gradually like you wrote in your post.
If any investor has money for investment, and wants to invest it quickly, DCA stragety is not a method to be chosen. It can be only one purchase or some purchases within short time like an hour, several hours, a day.
DCA method is for gradual investment with long term accumulation, and surely it takes a lot of time.