Well, one thing that the Cobras could do to keep their lifestyle going is set up a protection racket which makes the raiding lifestyle safer and easier for everyone.
They most likely protect their territory from rival groups already, so they can go to each settlement and trader, telling them that for protecting the area the people need to pay a tax. Pay the tax and everyone is happy, they don't pay, well... bad things will happen.
Now, most people will pay because this large raider group could do a lot of damage even for the strongest settlement. Even if the walls and militia are too strong, the Cobras could attack every trader, traveler and work party that is out of sight of the walls. Paying the protection money, as long as the Cobras don't get too greedy, is safer for everyone.
If a settlement is too poor to pay up in food, tools, supplies or horses/vehicles, they can pay with people. In fact, they'll probably demand some people from the weaker settlements. Depending on how the society works, the larger settlements might hand over criminals, debtors or malcontents to the Cobras so that they don't have to pay anything of real value. If these people show they're worthy they can join the gang officially, if not they stay slaves. A few people might volunteer to be tribute just for the chance to join up while sparing their community the tax.
There may even be a few settlements that base their economy around the Cobras. The Cobras come in with their wealth and they can gamble, drink, sell and buy slaves, and whatever else they want as long as they pay. Not paying would make the townspeople repay the damage in kind while the Cobras in question are in a drunken and drugged up stupor.
So long as the Cobras have the manpower, keep other raiders out, and don't act like wild dogs getting enough people to oppose them would be difficult. They would be the local gang that everyone grumbles about but everyone expects someone else to deal with.
With that kind of racket, there is no reason for the Cobras to settle down.