First thing you have to understand is that the PMP is a theoretical test based on the PMP, so even if you have 25 years of project management experience, you can still fail, and even if you have no experience, you can succeed if you study well.
Some project managers reported that a one month of monk-style studying is efficient (e.g. they go to a cottage somewhere and spend the whole day studying).
Doing some online tests is not that efficient, and most of these tests are manipulated to be very hard so that you can feel obliged to purchase the material sold on the website in order to help you "succeed".
Going back to your other questions, focusing on just planning is not enough, while some parts such as closing and monitoring do not weight a lot in the test, it is very important to know by heart all process groups.
I'm not able to recommend prioritizing certain process groups over the others, and if you really want to become a PMP, then you should study well, and if you fail, then it's not a problem, you can still re-take the exam shortly after.