Not a native, but I think "wonder" has a slightly different, more specific connotation of a subjunctive sort of thought (i.e., "could be" or "what if" rather than "how it is"), and думать is a more neutral sort of "thinking."
Of course, there's always the option to suggest it as an alternative and see if the crew accepts it.
It's much more common to think of someone else's life or think of an event in the past, unless you've been totally negligent about your own life and you suddenly realize: omg I'm so screwed up, then you would use "I think of my life"
Yes and no. The past imperfect tense in English will usually map into the imperfective aspect in Russian, but the past perfect tense can correspond to either imperfective or perfective. E.g. "In those days I thought a lot about life" would use imperfective думал, because it refers to habitual activity, whereas "I thought about your suggestion and I've decided to follow it" would use perfective подумал, indicating that your thinking came to a definite conclusion.
I agree, particularly with the yes and no part of your response. Reverso conjugate shows 17 English verb forms for "think" yet only 8 in Russian for думать and 6 more for подумать. I guess that is just more evidence that one cannot simply map grammar from two languages word for word.
But at this level of language study it is likely best just to understand the concept of ongoing (habitual) vs. completed actions. In the Russian to English version of this question, the use of Думал suggests that is all that is being taught.
Interestingly, I was going to write this comment using the words "English past imperfect" but I noticed "imperfect" is not used at all by Reverso, which prefers just "past continuous" for what I would have called "past imperfect". Yet they do define the past perfect continuous aspect, which I think would work here as well (but I need the assistance of a expert for that).
I expect "Я думал о жизни." would translate to either:
Past Continuous: I was thinking about life.
Past Perfect Continuous: I had been thinking about life.
So while it is not 100%, it seems you and I share the opinion to use perfective подумал, to indicate when one's thinking had come to a definite conclusion.