Advice on how to become a better candidate for the U.S Marshals Service

Or other jobs such as TSA? A distant uncle of mine who is a police commander in Wisconsin told me if I wanted a Fed job, I should start with something like TSA,

My ex girlfriend is TSA. I met a lot of people through her that jumped into TSA with the same idea as what your uncle suggested.

The nicest way I can put this is that I think you would be really unhappy with the job and the people if you decide to be TSA for a little while.
Do some type of local law enforcement...actual law enforcement experience will also help you for USMS, just doesn't stop the clock. But, be might have fun and decide to stay local...that's where the action is anyway. You go feds and you won't be any type of first responder.
I am waiting to apply to the USMS. I have a friend that is the Marshal recruiter in my area and he’s given me a well rounded run down of how things move along.

I have 4 years in the Marine Corps and a Bachelors. He suggested I complete a Masters of Criminal Justice as I await to start the process. He said it may help with hiring, but will definitely help with promotion later on.

There is a college in my town that offers online MCJ and MBA programs.

Any advice/thoughts on pursuing a Masters while I await to apply?

A Masters will help. A Masters, with LE experience will help more.
In addition you might look into the Intern Program that the USMS offers. One of our sub offices currently has a prior Marine who is attending college at this time as an intern. I know of several interns who were able to get employment with both federal and or state and local law enforcement. I may also add that you should apply with other federal agencies in the career field of your choosing. For example, the career field series 1811 has the same definition and requirements regardless of which agency you apply. Criminal Investigation Series 1811 Good luck and be prepared for an extended waiting period as the employment process normally moves quite slowly.