To destroy an outpost, I suggest instead of destroying the guards you need to kill the "Outpost Master" (whom you can buy new guards, maybe). This guy could have like 1000+ hps and be in a separate room inside the outpost (locked door/blocked by a guard), and surrounded with blocked tiles to prevent him from running away.
Not much to say on this old debate that needs comments from both players and developers. I'd say that it NEEDS to be a feature for NPC factions, and not for players gangs. Players gangs can do town control, and that feature would be the first step to make a NPC faction useful.
In a more general way, I think reward should be longer to come in, and that you could only create maybe two outposts for your (NPC) faction. This way, we will have outposts that
last longer, with a middle-term role. They need to last longer to attract newbies there (And it's easy, just put in a brahmins pen or a caravan boxes lifting job, maybe a workbench, and you'll get plenty of guys coming), and to have (If we keep the "Outpost Master" idea) more guards but with low HP, guards that wouldn't attack anyone from the concerned NPC faction, whatever they do. Attracting players there would be needed for the outpost creators, because they always need more players in their faction. By making a safe and interesting outpost (with huge PvP from times to times

), they make their faction more attractive and can hire new players in it.