When you have a population that has exceeded your needs for the pure survival aspects of the fortress, it's time to devote some effort into setting up your industries.
1. Create your military. Have a rotating three month schedule with one month training, one month patrolling, and one month off duty. Spread your veterans through your squads so the training goes faster. With three groups of squads, one will always be patrolling your entrance, one will be off duty but easily accessible for quick response, and one will be training to improve your squads. Ensure you either have the binary fix for long patrol thoughts, or have them assigned some very nice dining rooms and barracks.
2. Create your armorsmithing and weaponsmithing industries. Assuming you have access to wood, clear-cut your map and haul it all inside. Then have your wood burners work a dozen workshops on repeat until you have plenty of charcoal. Use this to repeat build large axe blades for training weaponsmithing and chain leggings for armorsmithing. Due to the way these are melted, they will actually return 150% of their cost in metal. Repeat melting these down will return 3 bars for every 2 used. It's something of an exploit, but allows you to constantly train these skills with a steady supply of fuel. This will ensure you have a well equipped army when the next siege arrives.
3. I personally like to smooth over all exposed walls in my fortress. Non-essential dwarves are given jobs doing this to keep them occupied. This might be a disadvantage if you plan to engrave later, as low skill will mean lower quality engravings. But skill level has no effect on simply smoothing, so feel free to have some useless migrants keep busy doing this.
4. Crafting trade goods from caravans and reselling them back can occupy a good number of useless immigrants. Enable leatherworking and clothesmaking and turn those bins of leather and cloth into trade goods. This trains legendary clothiers and leatherworkers quite quickly, meaning any leather armor you make will be far more likely to provide protection, and opens the possibility of using clothesmaking to create masterwork adamantine cloth cloaks for the ultra elite of your military.