The major thing here is to have that fixed mindset that you are actually planning on holding your Bitcoin investment for a long term and you are settled with yourself on this then starting up your strategy in achieving this won't be any hassle for you again. The DCA method no doubt is best method is you want to be convenient and steady with your buying even if it's now you buying it won't be a bad time because your plans are not to temper with the coins till like 6-8 years and if you steady in buying then when the time comes to this fixed period you will definitely smile at the achievement and how far the supposed investment would have grown.
You are correct, but let's not also forget that keeping that decision alive is based on the investors finance status. If it's the type that doesn't have much and is just managing based on earnings, if that earnings is cut short, there is a higher chance that they will, first of all, stop their regular accumulation process, and if there is a need for money to be used, it might also be tempered with. In such a situation, it's not about being determined; it's about making use of the only available resources. Sometimes this is what happens to some investors that make it look as if they don't hold onto their own decision.