people have said that it helps alot to get the perk that has the chance of producing mutagens when you make potions. haven't tried it myself. Have only come across one greater mutagen (which as I understand it is the only type that even count) and I'm halfway through chapter two.
The whole not letting you upgrade mutagens later is a real mistake with the game design. There is already enough vendor trash.
I don't see the point in farming for greaters. It takes way too much time to farm for them and the benefits don't seem worth it. Even on Dark Mode, by Act 3 your character should pretty much be a god without any mutagens.
Look up mutagenology on witcher nexus if you want to be able to combine mutagens into bigger ones. Even with mutagenology though expect to spend a loooooong time farming if you want to get useful greater mutagens. If you want all madness mutagens, forget it as it takes 3 of each of the major 3 greater mutagen types for one madness mutagen. I would estimate about 35+ hours farming the nekker pond, as it took me 5+ hours farming nekkers just to get the small amount of power and strength greaters I wanted.