NO, not JUST military service, PUBLIC service like picking up garbage or some other low-level but necessary community service! Even if one was a quadriplegic, the volunteer had to have a way to become a citizen with the voting franchise. IIRC, the easier the job, the longer the service requirement.
My recollection was that those who volunteered were not spared the risk of life or limb, even if they were disabled.
In fact, there was the episode with the merchant marines who tried to start a fight with the service men because they resented the fact their jobs did not qualify for citizenship.
I’ll back you up on that. Non-citizen civilians can enlist for Federal Service and try for a career field or one of a slate of positions they can choose from. If applicants score high enough in aptitude tests and there’s a need, they might get their dream job of plumber or starship pilot or...anything. (It takes a lot of folks in different fields to support a dropship and its complement of Mobile Infantry.
If there are no positions available in your chosen field. They’ll place into a field or specialty that has a need the applicant shows an aptitude for. Some jobs are easier than others, but no job is meant to be a cushy gig. The Federation might push a plumber off to Pluto to keep the pipes from freezing out there, for example—because that’s where the need is and Terra is full up with plumbers.
Once an enlistee passes their training, serve their term honorably (however long or potentially arduous that might be), and withdraw from the Service, only then can they receive their franchise and become full Citizens with voting rights.