Apis mellifera carnica build up is extreme. No other Apis mellifera to compare with.
Because of intensive build up, swarming is the hive management issue and not a genetic issue.
Adding new brood box or/and honey super in time is a must. You can add a new box before they need it if necessary – place a sheet of news paper between a brood box and a new brood box.
Swarming is the only natural way for the bee colony to reproduce. Turn that to your advantage. Carniolan bees are very good comb builders. They can build up to 30 frames in a good season.
Feeding only sugar patties won’t start the build up, it will just spend their strength. Pollen is a key.
Unless you go for very early honey flow, there is no need for extra feeding.