bridges:
best is to give every bridge it's own lever so you can controll precisely what bridge goes up or down. Note that you can NOT have a lever raise one bridge and lower another at the same time. It doesn't send a switch signal, but an up or down signal. (b) (g) selects the bridge, waxd selects which direction to raise, s sets is to withdraw (nice for pits sometimes). I make bridges as wide as the corridor they need to block and 2 tiles long. Why 2? well, 1 would do, but with 2 it's easier to see quickly if it's up or down. Longer bridges are only really needed to span pits or for atom smashers
My farms are 2x3, about 15 plots in total, plus above ground farms, also 2x3, 10 total. 2 make plump helmets, 2 pig tails, 2 cave wheat, 2 whip vines, 2 sun berries and 2 strawberries. That is all I need to feed a 120+ fort and keep it boozed up, but I got pretty good farmers. Fiddle around with it yourself, if you get too much, you can always make meals. My advice is to keep plots small so you can switch to different crops as you want. 4 3x3 plots is the same as a 6x6, but easier to split up to diverse production
caged prisoners can be disarmed by (d), (b), (c), (that is actually the default), then (d), (b), (d) to dump them, then make sure the cage itself isn't dumped, (k) if it's in stockpile or (t) if it's constructed. When you construct a cage, you can put as many prisoners in there as you want by assigning them. Thieves are supposed to have an escape chance, but everything else should follow the dwarf in charge docily. Prisoners make a good training for militia, as they skill up at least doubly as fast actually fighting than other ways
Thieves I usually scare off, not really worth getting upset about
And yes, it's worth upgrading armor. Iron is good enough for pretty much everything, steel is for if you can afford or make it, gives you a huge advantage, but isn't actually required
idlers:
First of all, set people to work at jobs you want them to do, not what they seem to be good at.
Second:
some people hate them, for one reason: they make friends so when they die, lots of bad thoughts. They are your ambush/siege cleaners, stone haulers, lever pullers, stockpile haulers and all that.
My young forts usually have no idlers. Growing forts have 10% idlers occasionally, developed forts have 20% idlers, big ones 30, mature 40 and my most developed forts can have as much as 75% of the population idle (at that point everyone is on military training). Most developed means that aside from industry to keep everyone clothed, boozed up and fed, nothing really needs to be done anymore.
Consider removing hauling jobs from master crafters, especially weapon makers for example. One of the cheesemakers can carry the axe he made to the stockpile, you want him to focus on making more masterwork axes