Buying Bitcoin at any amount at different intervals without it being the same amount is also good but it is better to use the regular DCAing of same amount at different periods of time because it will enable the investor to be more active and effective in the DCA but for an investor doing irregular DCAing, they might not take it seriously since they feel they can buy at any time they want to but for someone DCA same amount, they are more disciplined and are more likely to get huge accumulations since they are always consistent in their DCA amount.
It's not compulsory that you must DCA with a fixed amount as long as you are earning money and you are using some part of that money to buy bitcoin. That's totally fine.
There are times when you might have very high spending months in which you will have little left with you to spend on bitcoin investment, so at that 's totally fine, which you can also increase your monthly buying amount. It all depends on your finances at your point of buying.