Thing is, Mokuton Sashiki is actually one of the most hax Jutsu in the manga. If Cerberus is pierced by multiple branches, Obito is able to make them grow by a considerable amount remotely. I'm all for Cerberus hax, but you're actually saying it can fight without consequence with its head looking like
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. I find that highly unreasonable, considering Naruto canonically injured one with just a Rasengan [
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].
Where did it say Obito's anger amplified his Mokuton? It didn't. And the Zetsuit was only compensation for what Obito's weakened body (which he had just destroyed trying to break the boulder) was unable to do at that time. The Zetsu is perfectly integrated into his body now, that's why he can use Mokuton, replace limbs and freely manipulate Hashirama's cells (to control the Juubi).