... because involuntary service does not equal a person being property or a ward of the state.

I kind of feel like it does. If you are not allowed to leave or terminate service. You are owned.
I seem to recall Chakotay physically assaulting a crewmember (and also tells him that he's going to do it again and again and again) just because they wanted to do their job and not "jump through Starfleet hoops".
I'm not too familiar with every interaction on
Voyager, & if Chakotay is being abusive, that's problematic as an XO...
However it's still not forced service though. It's forced
standards of service. Not the same thing at all. I can try to do my job however I want, or more likely, the employer I agree to work for demands I do it their way, per my agreed employment. I'm still not forced to be there.
Dude could've just told Chakotay, well if I can't do it my way, I quit, turned over his badge & hung around with Neelix cleaning pots, until they dropped him off at the next available port of call.

Or he could've maybe made some other arrangements to stay aboard as a civilian. However it played out, he's not forced to work for Starfleet. He's forced to do things the Starfleet way, if he agrees to work for them.
Back on the OP topic... You know, Maddox was kind of an idiot too, in bringing the official litigation to begin with. By the end, everybody knows what his game was, & he not only loses out on his claim, but his defeat leads him to rescind the transfer orders too... possibly because he began to see the wrongness of his actions
The much more sinister move would've been to just order Data transferred under his command, which he couldn't refuse without resignation. However, Data only resigned when he found out the scope of Maddox's research, which as a commanding officer, he's probably not compelled to divulge to subordinates. So he could've just not told Data anything. Data would've been too clueless to know he should resign, & once he was serving under Maddox, then he could've incrementally pressured him with more infringements. The black & white line of liberty is then blurred more.
That's how the real world dirtbags work it.