Unfortunately not a modder myself, but I wanted to pose the idea.
Upgraded version of Robust Genetics: Genetic Memory
when your character is about to undergo a mutation, they are given a list of options:
Allow, the mutation to proceed as normal
Influence, taking on a mutation that your character has had previously that arent currently active.
Purify, similar to 2 but you use the impending genetic change to remove a mutation you have, or change it to the previous level (Reptilian Muzzle to Snout, for instance) EDIT: this cannot remove any mutations that cannot be removed with purifier.
Suppress, completely nullify the upcoming mutation.
I like the idea that your character can become accustomed to mutating to the point that their body gains some instictive control over the process, but its also really damn strong, so balancing suggestions would be appreciated. Maybe it can only be gained from having been mutated a certain number of times, and trying to control the mutations have some drawback like causing pain to your character, especially suppressing.