I know this thread is a month old but just throwing in my two cents of personal experience:
If you get a chance to grab an NEC as a HM then take it while you still can, they are becoming few and far between due to overmanning. Like wise for any FMF tour available once you finish A-school. As Scuba said you can always volunteer for SOCP while in A-school and FMTB. However don't fret if you don't get orders at that time either.
I would personally suggest seeing how you enjoy being a line corpsman while FMF to see if you have a passion for the field. I had the opportunity to do 4 years of hopping around infantry battalions in 1STMARDIV and didn't realize how much I loved the world of emergency and ditch medicine until then. A lot of people I was around who wanted to be a SARC or even just line corpsman discovered that they hate it and would rather be in a clinic setting, which there is nothing wrong with either. While I was in 1STMARDIV it was actually quite easy to 'transfer' to the SARC pipeline providing you can put down the PST scores. Due to my own mistakes though I've ended up surviving two NJP's now which leave me temporarily ineligible for the pipeline. It will be harder to secure orders to SARC while at a shore command, but again it is NOT impossible, that is what I'm doing right now.
I waited an entire enlistment plus more to find my self eligible again for SARC and just finished my package, 5 years later
Don't think of it as wasting time, think of it as more time to develop yourself into the most success-bound person you can be. I got NJP'd for that very reason when I got to the current command I'm at. Thinking, "Fuck this place and everyone here, I'm stuck here folding towels this isn't what I was made to do." That mindset had me Angry at everyone and ultimately in trouble with The Man. Like Scuba said again; no one looks for that mindset in a SARC or even line Corpsman.