BLUF: talk to PHS officers at FDA to get the ball rolling on this. In general, your application should focus on the mission of the agency, i.e. to raise the physical, mental, social and spiritual health of American Indians and Alaska natives (AI/AN) to the highest level. I would include any experiences you have working with Native Americans and/or diverse patient populations. Your experience living/working in rural communities is a plus and you should definitely highlight that on your application as well. Any public health/community health experiences are also worth highlighting.
You'll probably get better/more useful answers to your question by asking your FDA preceptor to connect you with PHS pharmacists at FDA who used to work at IHS prior to transferring to FDA. There are several such PHS officers at White Oak. In addition to getting better information, you'll also start working on developing your professional network within PHS, which is important both for successfully navigating the application process and for starting your career off on the right foot. Your time on rotation with the FDA (again, assuming you're at White Oak) is an excellent opportunity to build relationships with PHS officers that can help you through the process.
Applying for a job with IHS and for a commission with PHS are two different processes that are arduous, confusing, and require patience, resilience, determination, and acumen in navigating complex bureaucracies. It is competitive in the sense that it self-selects for the those who can figure out how to put all the pieces to the puzzle and are able and willing to see it through (not everyone has the money, time, or patience). The best way to increase your chances of success is to talk to as many different insiders (i.e., PHS officers) as possible. You will often get conflicting information - so trust but verify. If someone tells you "no", ask someone else or figure out a different way to ask the question. A lot of times people say something isn't possible because they don't know the answer and they don't want to promise something that will get them in trouble down the line. Do your homework as well. Scan through the CCMIS website, the PharmPAC website, and the USPHS website. Feel free to reach out to random PHS officers you see at conferences, or even on LinkedIn. We are used to random strangers talking to us - it goes hand-in-hand with wearing the uniform.
My understanding is your best chances of getting a commission with PHS as a pharmacist is to first apply for pharmacist jobs in IHS as a civil servant, and once hired as a civilian, then start the process of converting to the Commissioned Corps. Again, do as much online research as you can (honestly, there isn't a lot out there, but there is some good information publicly available), talk to as many different officers as you can at different levels of the chain to get the intel you need to successfully apply. As long as you meet all the basic requirements for employment with the federal government and for commissioning with PHS, the only thing truly limiting you (both from getting a job with IHS and from commissioning with PHS) is your sense of determination and how long you're willing to wait. You will likely have to apply to several different locations/duty stations several different times, wait several months until you hear anything back, and still not be successful. This is where having a support network and having PHS officers you can lean on for help is really crucial. Or you may just be lucky and apply at the right place at the right time and get a job offer within a few months the first time around. How your path is shaped often ends up being about having the right timing.
Disclaimer: everything I've said here is my own opinion and not that of my employer.