Its better to RETIRE a fort than abandon it. You'll still get migrants, and you also have a chance to revisit the last fort. I definitely seem to get a disproportionate amount of dwarves from previous forts, though, including ones that have already mooded. Especially the jackasses that were possessed! *sigh* those don't even have a high skill level to show for it, considering they don't also bring hte artifact with the mand are no longer qualified for another mood.
As for threats, mechanisms do sell well and train a valuable skill, but you can do other things. I like to make bone crossbows (one of the few good things bones are for), and then I sell off all the non-masterwork ones to the caravan. What bones to use? Just butchers your pack animals for starters (they are never worth keeping, even to breed later). Cage traps into the caverns/throughout the wilderness, hunters, and bringing cheap animals (like turkeys) on embark just to butcher all work. Or just use wood. If you have coal/magma and get the forges running soon enough, you can make them out of something useless like copper. I just prefer not to waste even junk metal on crossbows when they fire the same regardless of material. Sure, copper ones bash better, but do you really want ot rely on your RANGED units bashing?
Some people exploit other overpriced items... like wooden spiked balls. Here are these nearly useless, low quality, trap components GIVE ME EERYTHING YOU HAVE FOR THEM! Prepared meals are also just obscenely expensive. A lavish meal gets a value modifier for each ingredient, then a final one applied to all 4 when they get combined. Thus, you get food stacks worth more than ARTIFACTS.
Note: If your embark has decent metal ores you don't need to generate wealth to buy anything.
For general defense, two dogs chained (or at least something pastured) at your entrance works as a stealth-defense. Goblins and kobolds will usually just run when your dogs start barking at them. A bridge in front of these is also mandatory. A retracing bridge serving as a "door" to your fort is one of the first things you need to set up- before the dwarven caravan arrives even. This includes the lever hooked up to it in a place like your dining room for easy access.
Other than that, cage traps are also overpowered. A rickety wooden cage with a no-quality mechanism will capture a ginormous FIRE BREATHING dragon 100% of the time. Ironically, the dragon can melt the cage before stepping on it (if shooting at another target), but not destroy it once its caught. Some people refuse to use them for this reason, as 100 cage traps will 100% stop that goblin siege. I generally just leave them behind my bridge to the caverns to capture food and breeding material. Mmmmm cave croc breeding programs! Jabberers are also war-trainable...
Don't over-equip a fresh squad of soldiers, either. Armor user skill reduces encumbrance from heavy armor, which reduces both movement AND attack speed of soldiers. Thus, you start out in leather/bone armor, or give them only a few light pieces of metal... like helms/gloves/boots. Then use leather armor and maybe bone greaves. Metal breastplates/greaves are definitely out. I think a mail shirt can be allowed, but I'd rather not if it was copper (heaviest armor). The shield will definitely be wood. Wood and steel shields block the same (even dragonfire!), so weight only matters for shield bashing. Your mighty and legendary armor users can pick up heavy shields, your recruits will manage with wood.
There are also more advanced forms of defense... since you like to set up near waterfalls and rivers, you could easily set up a drowning chamber. Make a (lever-controlled) channel into the river above you. Have bridges raise to seal off a section of your entrance and trap intruders in it. Then open that hatch/bridge/floodgate above and fill the space up with water. Dig a drain (or let it drain back out the entrance) and dispose of the bodies/recover your goblinite. Remember... amphibious and non-living enemies won't drown. This will NOT work on zombies. You can take this a step further to make a "freezing" or "obsidian casting" chamber. There are also minecart shotguns, grinders (full minecarts on a loop to crush enemies), and siege engines.