That's right, we can collect Bitcoin whenever we have money. We can buy Bitcoin using the strategy you said. Using this strategy is very good because we won't realize that we already have a very large amount of Bitcoin. Of course we buy Bitcoin using money that is not being used so it won't make us feel stressed and make buying Bitcoin a way to save in the future.
You will not find an opportunity but you have to make an opportunity by yourself because if you are waiting for the opportunity then you will miss the better time for investment and maybe when the time arrives then you will have to spend that amount on other things.
It is hard to predict what is the highest worth and what is the lowest worth therefore try to utilize your money whenever you accumulate it and desire to invest it as interest is crucial for goals achievement.