I personally think that those who invest in Bitcoin should take sound and reliable decisions for their investment.
Of course, every investment has a risk, even if you are ready with all the risks you take. That's why we have to use reserve money in investing because we know it's cold money that we won't touch in the long run.
Everybody has their own strategy but in this context I think when you use your spare money, isn't it a deterrent for unexpected needs?
I personally prefer when budgeting every monthly income to be used as investment money. Because in my opinion when using spare money then this possibility can slightly hamper when there are urgent needs but when budgeted in advance to invest, for example like 10 percent every month then apart from you can know how much your capital is to invest, you can also use your spare money for other purposes such as unexpected needs or prepare to face some chaos such as inflation etc.
It can be a good thing when you can allocate a few percent of your income to be in bitcoin.
But as you said before, everyone has their own thoughts and strategies, when doing it with spare money as long as it doesn't interfere then I think it's also good, it's just that this should pay attention to more conditions where when there are unexpected needs then they should have some policies to make their finances not disturbed in terms of plans.
But when it comes to comparison then I prefer your way with budgeting beforehand apart from we know how much we spend on investment, unexpected needs can be overcome because there is an allocation there in financial management.