The other 2 were also desperate. They didn't bash your head.
One had only been enslaved for a few weeks, and the other believed her master loved her; neither were nearly as desperate as Nia, who has made clear that she's suffered under some particularly bad masters for decades, if not centuries. It's also worth noting that both Ann and Sylvia sat in the van while Nia came to get you. If Sylvia had been the one who came to drag you into the van, she might just as easily have resorted to violence.
You would think the one that was just introduced into the slavery for the first time would be the one more desperate
Why would you think that? That's like saying a person who's been in prison for a week (and hasn't had the reality of their situation sink in yet) is more desperate to get out than someone who's been in for twenty years, and who knows escape is the only way they're ever getting out. While both will be eager to escape, it's the latter who would be more desperate.
If you pay closer attention to Silvie's dialogue she mentions she had escaped before it wasn't her first time as a slave.
Yes, she did say that, but she was lying. She mentions during Lin's most recent event that she wasn't telling the truth, and had in fact only arrived on the continent three months ago; she thought she'd blend in, realised that wasn't the case, and then got caught by Cornwall.
Interestingly enough, even when admitting she'd lied in the past, Sylvia still continues to tell new lies; she claimed to have left her home of her own free will, and then a minute later says she can't return home or she'll be imprisoned, if not killed. She lies a lot, mostly out of pride.