Well, if you want to avoid FBs, you can always either wall in your cavern, or just the staircase room that carries you below. If you reveal cavern 1, you won't get FBs in caverns 2 and 3 until they are also revealed (though I've had those caverns interconnect through pits and revealing one revealed all 3, but that's somewhat rare.)
So, I'd say to enlist an army of masons, crank out blocks and bins as fast as you can, get them staged, breach the first cavern, send in the army, and start walling off. Technically, you can seal up right to the edge and prevent them from ever spawning, but that's a hell of a lot of stone. Instead, go for choke points, and be careful to get all of the little corners where a flier can sneak in. You can also do it in stages, get a minimal perimeter established with a drawbridge to give you a safe way out, look for the next opportunity to expand, go for that, and repeat while it's safe. If a FB shows up and you can take it, then go kill it and continue. If it's the poisonous vapors made out of steam or steel or some wicked thing, then call it a day until you can build a proper trap for it, and keep digging down.
I wouldn't worry about running out of enemies. Many of us have run forts for 50-100 years, racked up thousands of kills in individual dwarves even and never suffered a shortage of things to attack us. As for wildlife, things may have changed from 40d days, but I thought I extincted all of the local wildlife in one fort only to find a few decades later that new ones spawned. That was a bit of conventional wisdom overturned, but it really depends on how dedicated you are to this fortress. I don't think many players can drag their games out that long, if for no other reason than the game starts to lag quite badly.