If you don't want to spend time preparing for the UMAT and concentrate on achieving the highest ATAR you can get then just do the UMAT even if you don't practice just in case you change your mind later! Practice isn't really essencial. I can give you a very simple strategy to how to do the questions:
Section 1: Read the options before you read the passage so that you read with purpose and choose the answer that is clearly supported by the passage.
Section 2: Read the passage carefully and then read the question without reading the answers, think about the answer then read the options.
Section 3: This is IQ, it is the easiest section IMO.
This is a very simple strategy to doing well in the UMAT so just do the test! Medicine is an awesome career!
For your other subjects, start doing past papers while learning the content. Try to get 40 past papers done before the trials and about 60 before the HSC.
Good luck