It's nothing like splicing anything. he is morphing himself. that wasn't some random Grumpkins cock he was fucking Liara with, it was his own that had been altered to look like a Grumpkin, that wasn't a random trolls cock he fucked Succubi daughter with it was his own... the only difference between what I suggested and what the MC has done many times over is that he is reducing the amount he needs to transform.
Who said it would require no resources, Energy is a resource and I can't imagine magic that doesn't consume it. The advantage of potions being that they preserve your energy levels rather than expend them. Leaving both options as valid depending on how you balance your character.
Tinkerer could also add a food requirement, it might be nice to finally have a use for it. (other than a couple of times at the very beginning of the game I've never felt the need to eat

)
And frankly I want to see the MC grow, it's has been a long time since he learned anything worthwhile, and developing his shapechanging abilities in a more proactive way seems like the perfect way to go. He could apply it in battle too (Though TBH I don't really battle, I cheat my stats to skip those)