I don't really think that buying a fix amount of Bitcoin when talking about DCA is proper because to my own understanding, if you really want to buy through the DCA method, then you have to buy with a fix amount of money, weekly or monthly irrespective of the current price of Bitcoin, but if you want to buy a fix amount of Bitcoin, them you might spend more or less depending on the current price of Bitcoin, but the major challenge with this strategy is that you will be forced to tamper with you emergency fund that would have been for your upkeep till month ending when you are expecting money from another source.
I just believe that the DCA method is design for us to buy in a comfortable manner, not in away that will make us to start doing opportunity cost just to meet up to the target of buying a fix amount of Bitcoin.
Let's understand this DCA stuff, when you are DCAing it doesn't mean that you are stagnated or forced to continue investing the particular amount you have been investing, you can decide or be flexible enough to adjust or increase your investment as you desire it to be, so don't misunderstand this process, I must say that your perception about this strategy is wrong, as far as Bitcoin investment is still in existence DCA is one of the best strategy rolledout for everyone no matter your income to be involved in Bitcoin investment, no one is going to do any opportunity cost stuff to meet up for anything as you said, what everyone that's is involved in DCAing needs to do is, keep investing the amount that will be comfortable for you so that you wont go about trying to borrow or thinking about spending your emergency fund in other to satisfy your investment plan.