If you want to have a high score, it is a good idea to start with your preparation as soon as possible. That way, little by little, you will get that material under your skin.
I cannot tell you which specialty is right for you. I think that most people don't have a clue what is right for them, me included. So I guess, you should do what feels right.
You never know, how will you end up feeling about the thing you chose. Maybe it will be great, maybe you will want to run away. What you can do is: talk to people in the specialty you're interested in and get as much experience as you can. That way, you will have a better idea if you like it or not.
As for the competitiveness, NBME uploads match statistics, one of those tables has average Step 1 score (and probably there is one for step 2 ck too) for matched applicants with interquartile ranges in different specialties. According to that table, the mean score for matched applicants in psych was 220.
IMGs do get into competitive residencies, i know a couple of people who did get into very competitive programs. However, these people are crazy in a way. What I mean to say. If you are really determined and hard working, there is a good chance that you will get into any program you like. But it is not that common. If you want to pursue a super competitive specialty, you should have a very strong CV. That means - good scores, LoRs, US clinical experience, research publications, etc.