Undergraduate vs. Masters Level Classes To Show GPA Improvement

Hi!
I am a nontraditional student:

  1. I completed undergrad but did not do well too well, particularly in sciences (life events and I was not a great student).

  2. Completed a post bacc, showed an upward trend but did not do a stellar job, below a 3.5 in post bacc (this is really the time, I was learning what it was to be a student).

  3. I have have the following: research, letters of rec lined up, extracurricular, volunteering, clinical experience, shadowing, essentially a well-rounded candidate, not just checking boxes but doing things I enjoy and learning.

I am planning to apply for 2021, thus this cycle and am scheduled to take the MCAT this summer (1st time- I have been preparing (I have chosen a date that will have expedited score processing)).

I would like to keep taking classes, post MCAT, thus this fall and even in the spring, to show schools and myself that I can continue to improve and learn to master the sciences. Thus I am wondering, is it helpful for me to take undergraduate-level courses to show this or master level courses? During my last semester of post bacc, I did take 1 master level science course (biostats) while taking undergraduate level biochemo (within the post bacc) and did do well in both courses.

Essentially, is it better to build on my new “graduate GPA” or my “undergrad GPA”?

Any help in the right direction would be helpful, please.

Thanks!