DCA approach can only be applied to bitcoin, this is because bitcoin has great potential and is likely to be more valuable with the timeline as long as you are in a long term investment. Another thing you should know is that DCA approach can be done by both the pleb and the rich which any amount even as low as $10. It depends on how much your income is and how you feel is best for you to start investing with.
Bitcoin investment is also good and DCA method is also the best, but considering the price of Bitcoin we must think that it requires a good amount of money to invest. Those with less capital and those with more capital can invest in it, but the one who invests the most can get the highest profit. I don't think a $10 DCA can make a good future profit in Bitcoin.
One can do DCA according to the amount of income he has but choosing Bitcoin with less income I think may not be a good decision. If one has less savings then there are very good and strong projects in the market instead of Bitcoin where one can earn a good profit in future by investing through DCA. Altcoins can also be great for us so it would not be right to ignore them.