Should I retake a course for an easy A or focus on research?
PhD students are judged on their research publications, not their coursework.  It is good strategy to minimize the time you spend on courses.

we have no transfer credit policy

If there is no policy, ask for one.
I would recommend taking a low effort course over a high effort course; not to reduce effort expended, but to direct that effort to research.  If a course helps you with your research, then take it.
I would not recommend lowering your coursework efforts to the point that you fail; in that case you will need to repeat the effort, which is not efficient.