Seating problem
When I load cast bullets (using carbide dies) there is a crud buildup in the bullet seating die, but not (obviously) from brass sizing lube. The crud comes from the lube for swaging the bullets, and from the lube grooves on the bullets. A little always works its way into the seater die. And as it builds up, it changes my seating depth. I just adjust for this as the reloading goes along, unless it gets really bad.
The answer for me has been to take my die to my gun cleaning station after every session, disassemble it, and clean it with solvent. Annoying, but it works.
You do want to be careful with seating depth, as the amount of room for the powder to expand when it begins burning varies with changes in how far down the bullet's back end intrudes into the case. Change that room, and you change your pressure, and therefore your bullet velocity. Change bullet velocity, shot to shot, and there goes your accuracy. In extreme cases you could cause a dangerous overpressure.
You also mentioned case mouth belling. You want to bell your case mouth as little as will allow your bullet to sneak into the case undamaged. Less belling means less working of the brass when you crimp your bullet. Less brass working means less splitting of cases and more reloads per case. BTW, you also don't want to crimp the cases any more than necessary, for the same reason. However, heavy loads and heavy bullets require a heavy crimp.