As others have mentioned picked up a unit of infernal guard (28 strong) and add the bsb and prophet to make the unit 30 strong. Leave the hellsmith to hang out with the war machines. To make the changes you’ll have to drop lots of hobgoblins.
Fairly solid list, although i don’t think the hobgoblins add much (other then making sure u have the minimum core points). You may be better off taking a single larger unit rather then the 2 units of 29.
As far as character equipment goes you may want to take the dawnstone on the bsb. If you took the enchanted shield, dawnstone and hammer he would have a 2+ re-rollable save and still be hitty with the hammer in combat. With only 3 attacks though I don’t really like the hammer on castellans. I’d rather see you get some more protective gear on him.
I’ve considered trying the mask out on my prophet, but I just feel the talisman of preservation/healing potion is a much better combo for him. The mask is a bit costly for what it does, although the increased armour save is helpful. I’d drop the talisman off the daemonsmith and move it over to your lord. Throw a dispel scroll on the smith and keep him cheap. If he dies it isn’t really that big of a deal since I’m assuming he will be close to the war machines. I’m assuming you took the level 4 and level 2 upgrades?
I’ve been finding the iron daemon a bit hard to use on the flanks with its lack of movement ability. I prefer throwing him up the center of my opponents battle line with the destroyer assisting on its flank.