Okay, Thace would have abhorred the Overmind. On the other hand, extinction was an awful price for freedom. He wouldn't have agreed with a Traveller strike (but probably could have been kept in liminal if someone wanted to stop him acting via portal lockdown or something; portals was never his strength), and would have devoted all his time to finding ways to help the rebels survive after they got their freedom, and probably been encouraging them to play a long game, if they could afford it. (Store up supplies, techniques, etc to survive after they used the hellstone) Infiltration and stealing information to help the rebels is something he would have considered as a first course of action. He would have been very willing to die to help the rebels get what they needed to survive the coming battles they would face.