As I was typing before being so rudely interrupted by a crazy forum revision!!!
So from what you have told us so far, I have a couple ideas on “conventional” tactics in this world.
No gunpowder or explosives dictate that any projectiles not propelled by theurgy must be launched mechanically. That means slings vice muskets and trebuchets/ballista instead of cannons. Slings stopped receiving technological innovations with the advent of gunpowder. Obviously in this world that is not the case. I would postulate that an upgraded mechanically assisted sling could propel a fin stabilized projectile made of iron or lead with reliable accuracy and armor penetrating force (150-200 fps) up to 100 meters (the typical engagement range of volley fired smooth bore muskets). This comes with a cost in that a loose formation would be required to operate a sling and slinger infantry would probably not be able to carry a large shield or any shield at all. Unless…they employ a staff sling. So idea here would be that primary weapons is a halberd/billhook/pike. This weapon can separate in the middle and is held together with a chain. The base of the weapon is filled with these stabilized bullets (think nerf football with 2 squiggly fins attached to the ball itself and no tail). The top of the weapon is held as the bottom is swung generating centripetal force. When spinning with enough force when the weapon is jerked suddenly in the opposite direction a spring at the base of the staff is depressed releasing one bullet while stopping the remainder from falling out. The weapon then automatically cycles the next round with the spring is decompressed by the absence of the “jerking” force. This allows the infantry man to have a large shield in his off hand. Costs are that the staff would be weak in the middle and the weapon would be unbalanced until all the ammunition is expended. Each foot soldier would also have an arming sword for close combat. Heavy and light cavalry would counter the infantry which had to maintain a loose formation to use their projectiles. The best troops would be well drilled in quickly changing formation from skirmishers to square and back again. Armor for heavy infantry would have a full sloped helm with heavy steel paulderons that has loosely fitting scale mail attached that hangs down to the ankles. This would be best for reducing the inertia of slung projectiles. Obviously both the siege weapons and bullets would be accelerated or stopped by theurgy. It seems like complex materials are harder to control so maybe the ammunition is an alloy or made of many different layers of material. Theurges controlling the infantry would be able to identify the bullets easily since they are used to them while the enemy might have a hard time separating the bullets from other items on the battlefield.
Thoughts?