Is `方法是通过xxx` a correct sentence? i.e. 中文中,介宾结构能作为名词(宾语)使用吗
'through' implies 'going through' and  通过 is the correct translation for it. There's nothing wrong with the first sentence
When you 'going through' something to get somewhere, that something must have a length for you to go through (you have to spend a period of time)

通过反复练习  (through repetition) sounds right because 'repetition' requests a considerable length of time

通过自由、有意识的决定 (through free and conscious decision) doesn't sound right because 决定 (decision) happens in an instant.


The more reasonable sentence would be:

The third way is through free and conscious consideration
第三种方法就是通过自由、有意识的考虑
通过考虑 (through consideration)

or

The third way is to trust free and conscious decisions
第三种方法就是信赖自由、有意识的决定
信赖一个决定 (trust a decision)