They were quite literally prisoners of war, kidnapped at gun point during war time by soldiers who invaded Germany. They were managed by the same administrative body that oversaw forced labor. They were POWs by all definitions.
Yes, they were treated decently, because that's what is generally required to get people to do intellectual labor. The GULAG system generally did treat people decently, much better than the US prison system, that's for damn sure.
As for being released from the USSR and back to Germany, if they weren't POWs serving time in a forced labor arrangement, then why couldn't they go back to Germany until the Soviets released them? Their forced labor was their punishment. The Soviet GULAG system was literally founded on the idea that contributing your labor to society was exactly how you paid your debts to society. They were punished, they served time, and they were released. It's not like the Soviets brought military butchers back to integrate them into society, nor did they bring strategists and tacticians to integrate into their military structure. They wanted knowledge work, they got knowledge work.
As for the lackluster results, I'm sure you're familiar with the fact that there was almost zero rocket science happening in the USSR before and during the war. After forcing German scientists to train Soviet scientists, the Soviets developed an exact replica of the V2. I would say that alone demonstrates the success of the program.
But more to the point. They went from zero rocketry to the number two rocket science center in the world. Sure they didn't beat the Americans to a heavy launcher and the Saturn V became the gold standard. But the Soviets achieved over a dozen milestones before the US did with satellites, manned flights, probes on Venus, etc. If you think going from literally nothing to second place is lackluster, perhaps you have a bias.