You are right in that passive voice is more common and accepted in other languages, whereas in English it is shunned, often unfairly. Most English readers, including academic readers, prefer the subject verb object arrangement of sentences. This is not to say, however, that passive voice should be avoided at all costs. I think you will find that English readers are remarkably tolerant of passive voice if the prose is good, which is to say that one ought to be deliberate about the selection of "voice" in English. The key question is whether there is a good reason for the passive. For example, passive voice can be helpful if the subject is unknown. It can also be employed if the author does not wish to name the subject. Consider the delivery of these two notionally identical sentences.
Board members froze salaries for the third consecutive year because of languishing sales.
Salaries were frozen for the third consecutive year because of languishing sales.
Passive voice can also be used for a shift in emphasis. A simple example below shows how much more preferable is passive voice at times:
A ball hit my son and hurt him. (emphasis: ball's action)
My son was hit by a ball and is hurt. (emphasis: son's condition)
As far as the "I"-"we" distinction for your dissertation work, I would imagine that this might change depending on where you write your dissertation. I have seen both approaches. Personally, I find "we" a nuisance as a reader, but others prefer it. But then again, writing is often field-specific. One of the best things you can do as a doctoral student is to read (or skim) a couple of dissertations written under your supervisor or written for your particular program. That will give you a sense of the expectations. Of course, find out how those students performed on the dissertations to determine if they're worthy of your imitation. I hope this was helpful. Best of luck to you.