There are 8 Grishas active in the Pacific Fleet. They are building 8 Steregushchiy/Gremyashchiy ships for the Pacific Fleet. The Pacific Fleet has 4 Nanuchkas and they are building 4 Karakurts.
There are 2 Grishas in the Black Sea Fleet. They either built or are in the process of finishing 2 Steregushchiy ships for the Black Sea Fleet. Need I continue?
The Grisha and Parchim had crew sizes of 86 and 81 respectively. The Steregushchiy and Gremyashchiy have a crew size of 100. While that is larger, the difference in crew size is way smaller than the change in displacement where they have over twice the displacement.
It could be they simply are not in the mood for single mission ship types anymore. Where you tie down hundreds of sailors just on anti-submarine work.
From what I understand the main difference between the Buyan-M and the Karakurt is that the Karakurt is meant to have better sailing characteristics in deeper waters. The original Buyan had a draft of 2m, Buyan-M has a draft of 2.5m, and the Karakurt is 3.3m. While the first series was made for the Caspian Sea, the next series was made for the Black Sea and the Baltic Sea mainly, and the last seem to be made for the Pacific and maybe North Sea eventually, although is also being used in the Black Sea and the Baltic Sea. It seems to be a process of continuous optimization where they initially designed a ship to be as capable of working close to shallow waters and inside the canal system then gradually increased the draft so they could operate further away from the shore. Maybe the canal system now allows such higher draft vessels to operate where previously it wasn't possible. I know they have been dredging the canals over the past decade and in some cases changing the locks even.