If you count the Kontors on play, rather than at the beginning of next turn, then each Kontor you play will count the Kontors from the previous turn, the Kontors you've played before this turn, and itself. So, +4, +5, +6 if you have 6 Kontors, playing half each turn. If you count them at the beginning of the turn, then it would just be +3, +3, +3.
I feel like adding a buy restriction might allow one player to get them all unopposed, which then would clearly be too strong.