The annoying part is that there are a lot of named mutagens with the same stats as lesser but you cant combine tho into greaters. They should allow you to dismantle those into normal lessers.
Just to be sure I understand, there are very few Greater Red dropped, or do you mean there are rarely any level of red at all? I've had to combine low level ones into greater for green and blue, and want to understand as I level up what red is going to be like.
I envy youI just picked up 6 lesser red mutagens from drowners in Skellige. Almost every one gave red. That was just from a random walk around the coastline. Those were my first in the game, so I still don't know how much this was pure luck.