Sounds like you have 30 dwarves supporting the entire fortress. That *should* be enough as the military really only needs food/booze.
I've got a rather large sprawling fortress with 72 dwarves, only 32 of which are productive (the rest military, nobles, or kids). They have no problem feeding everyone (I do a 30 lavish/30 booze run every year or two and keep a 4x4 farm going all the time), and I can't do everything at once, but I have no problem keeping 1-2 dwarves on skilling tasks (I'm currently skilling up my gem cutter and glass maker).
The worst problem is when the military does their thing and I have to haul all of the gobbo crap. Simply moving where I engage the enemy closer to my trade depot has helped considerably. But I've only got 2 marksdwarves in training and 4 melee (2 legendary wrestlers, 2 legendary wrestlers/hammer) and they have no problem at all holding off the 3-4 ambushes I get each season. I've yet to face a legendary ranged gobbo, though. Been lucky there.
I only have one set of traps as last resort (never been used) though I do have a pair of drawbridges I can close to control matters somewhat - but one of them is always open. I should think you would have no problem at all with the force you currently have if you deploy them well.
The problem with draft armies is that they will be unarmed/unprotected when you deploy them. By the time a dwarf gets his armor on and grabs a crossbow and a quiver and ammo, the siege will have crossed a 3x3 map. Since you have no drawbridge to raise while everyone gets dressed, I'd just give up on that as a useful option. Realistically, expect them to engage unarmored and possibly unarmed (keep woodcutting on as a skill and at least they'll always have an axe on them) and expect them all to die. You have the added concern that any good draftee will hit elite status before you know it and you won't be able to reincorporate them into your labor pool. Dwarves skill up very fast in actual combat.
Instead, I'd add war dogs to your military in a major way - cage+lever at key points, that kind of thing. They can really tie up an attacking army while your regular soldiers do their thing. Rather than depend on a draft army to handle sieges, I'd look for ways to move the battle to your favor. If you don't want to be able to cut off attack altogether, create choke points that blunt the effectiveness of ranged units and force the enemy into a narrow area that spreads them into a long line but opens into a large area where you have freedom of movement and can overwhelm with force. Unless as part of your challenge you want to engage on an open plain, in which case my only advise is to breed more, because the first siege of 25 ranged units with a legendary ranged leader will cut your melee guys down in nothing flat unless they are all legendary shield users and very well equipped.