You can always go CID and then reserves while still working as a civilian federal LEO. I currently work with a AF OSI reservist who came off active duty recently. Similarly to what psalms is talking about, he didn't care much for the crim mission and focused on the CI side of things, at which AFOSI is/was unusually aggressive about unlike almost any other federal LE agency.
I don't know of any federal LE agencies that will take Ft Leonard Wood's investigator program as a substitute for FLETC CITP, so if you're sensitive to the idea of having to repeat academies then I'd factor that into your life choices. Some people are, and seem to have the life sucked out of them by training/academy settings......personally, I thought FLETC CITP was fun. It was basically college with guns, and virtually no responsibility beyond 1) Study and pass, and 2) Don't feloniously add or subtract from the population.