Instead of a monthly purchase I'd probably set a total annual amount I'd like to invest in Bitcoin. (I ain't that rich enough to regularly buy) Then I'd just sprinkle them out throughout the year during crashes.
Everybody should do whatever they think is best and what they feel comfortable with. I prefer the automation; if it's about long-term investing it's the equivalent of buying ETFs or stock regularly. However, never put any money in crypto that you can't afford to lose (at least up to this point).