set_option maxHeartbeats 400000 in
theorem tensor_associativity (X₁ X₂ Y₁ Y₂ Z₁ Z₂ : C) :
    (tensor_μ C (X₁, X₂) (Y₁, Y₂) ⊗ 𝟙 (Z₁ ⊗ Z₂)) ≫
        tensor_μ C (X₁ ⊗ Y₁, X₂ ⊗ Y₂) (Z₁, Z₂) ≫ ((α_ X₁ Y₁ Z₁).hom ⊗ (α_ X₂ Y₂ Z₂).hom) =
      (α_ (X₁ ⊗ X₂) (Y₁ ⊗ Y₂) (Z₁ ⊗ Z₂)).hom ≫
        (𝟙 (X₁ ⊗ X₂) ⊗ tensor_μ C (Y₁, Y₂) (Z₁, Z₂)) ≫ tensor_μ C (X₁, X₂) (Y₁ ⊗ Z₁, Y₂ ⊗ Z₂) := by
  have :
    (α_ X₁ Y₁ Z₁).hom ⊗ (α_ X₂ Y₂ Z₂).hom =
      (α_ (X₁ ⊗ Y₁) Z₁ ((X₂ ⊗ Y₂) ⊗ Z₂)).hom ≫
        (𝟙 (X₁ ⊗ Y₁) ⊗ (α_ Z₁ (X₂ ⊗ Y₂) Z₂).inv) ≫
          (α_ X₁ Y₁ ((Z₁ ⊗ X₂ ⊗ Y₂) ⊗ Z₂)).hom ≫
            (𝟙 X₁ ⊗ (α_ Y₁ (Z₁ ⊗ X₂ ⊗ Y₂) Z₂).inv) ≫
              (α_ X₁ (Y₁ ⊗ Z₁ ⊗ X₂ ⊗ Y₂) Z₂).inv ≫
                ((𝟙 X₁ ⊗ 𝟙 Y₁ ⊗ (α_ Z₁ X₂ Y₂).inv) ⊗ 𝟙 Z₂) ≫
                  ((𝟙 X₁ ⊗ (α_ Y₁ (Z₁ ⊗ X₂) Y₂).inv) ⊗ 𝟙 Z₂) ≫
                    ((𝟙 X₁ ⊗ (α_ Y₁ Z₁ X₂).inv ⊗ 𝟙 Y₂) ⊗ 𝟙 Z₂) ≫
                      (α_ X₁ (((Y₁ ⊗ Z₁) ⊗ X₂) ⊗ Y₂) Z₂).hom ≫
                        (𝟙 X₁ ⊗ (α_ ((Y₁ ⊗ Z₁) ⊗ X₂) Y₂ Z₂).hom) ≫
                          (𝟙 X₁ ⊗ (α_ (Y₁ ⊗ Z₁) X₂ (Y₂ ⊗ Z₂)).hom) ≫
                            (α_ X₁ (Y₁ ⊗ Z₁) (X₂ ⊗ Y₂ ⊗ Z₂)).inv :=
    by pure_coherence
  rw [this]; clear this
  slice_lhs 2 4 => rw [tensor_μ_def₁]
  slice_lhs 4 5 => rw [← tensor_id, associator_naturality]
  slice_lhs 5 6 => rw [← tensor_comp, associator_inv_naturality, tensor_comp]
  slice_lhs 6 7 => rw [associator_inv_naturality]
  have :
    (α_ (X₁ ⊗ Y₁) (X₂ ⊗ Y₂) (Z₁ ⊗ Z₂)).hom ≫
        (𝟙 (X₁ ⊗ Y₁) ⊗ (α_ (X₂ ⊗ Y₂) Z₁ Z₂).inv) ≫
          (α_ X₁ Y₁ (((X₂ ⊗ Y₂) ⊗ Z₁) ⊗ Z₂)).hom ≫
            (𝟙 X₁ ⊗ (α_ Y₁ ((X₂ ⊗ Y₂) ⊗ Z₁) Z₂).inv) ≫ (α_ X₁ (Y₁ ⊗ (X₂ ⊗ Y₂) ⊗ Z₁) Z₂).inv =
      ((α_ X₁ Y₁ (X₂ ⊗ Y₂)).hom ⊗ 𝟙 (Z₁ ⊗ Z₂)) ≫
        ((𝟙 X₁ ⊗ (α_ Y₁ X₂ Y₂).inv) ⊗ 𝟙 (Z₁ ⊗ Z₂)) ≫
          (α_ (X₁ ⊗ (Y₁ ⊗ X₂) ⊗ Y₂) Z₁ Z₂).inv ≫
            ((α_ X₁ ((Y₁ ⊗ X₂) ⊗ Y₂) Z₁).hom ⊗ 𝟙 Z₂) ≫
              ((𝟙 X₁ ⊗ (α_ (Y₁ ⊗ X₂) Y₂ Z₁).hom) ⊗ 𝟙 Z₂) ≫
                ((𝟙 X₁ ⊗ (α_ Y₁ X₂ (Y₂ ⊗ Z₁)).hom) ⊗ 𝟙 Z₂) ≫
                  ((𝟙 X₁ ⊗ 𝟙 Y₁ ⊗ (α_ X₂ Y₂ Z₁).inv) ⊗ 𝟙 Z₂) :=
    by pure_coherence
  slice_lhs 2 6 => rw [this]
  clear this
  slice_lhs 1 3 => rw [← tensor_comp, ← tensor_comp, tensor_μ_def₁, tensor_comp, tensor_comp]
  slice_lhs 3 4 => rw [← tensor_id, associator_inv_naturality]
  slice_lhs 4 5 => rw [← tensor_comp, associator_naturality, tensor_comp]
  slice_lhs 5 6 =>
    rw [← tensor_comp, ← tensor_comp, associator_naturality, tensor_comp, tensor_comp]
  slice_lhs 6 10 =>
    rw [← tensor_comp, ← tensor_comp, ← tensor_comp, ← tensor_comp, ← tensor_comp, ← tensor_comp, ←
      tensor_comp, ← tensor_comp, tensor_id, tensor_associativity_aux, ← tensor_id, ←
      id_comp (𝟙 X₁ ≫ 𝟙 X₁ ≫ 𝟙 X₁ ≫ 𝟙 X₁ ≫ 𝟙 X₁), ← id_comp (𝟙 Z₂ ≫ 𝟙 Z₂ ≫ 𝟙 Z₂ ≫ 𝟙 Z₂ ≫ 𝟙 Z₂),
      tensor_comp, tensor_comp, tensor_comp, tensor_comp, tensor_comp, tensor_comp, tensor_comp,
      tensor_comp, tensor_comp, tensor_comp]
  

Proof state:
C : Type u₁
inst✝⁸ : Category.{v₁, u₁} C
inst✝⁷ : MonoidalCategory C
inst✝⁶ : BraidedCategory C
D : Type u₂
inst✝⁵ : Category.{v₂, u₂} D
inst✝⁴ : MonoidalCategory D
inst✝³ : BraidedCategory D
E : Type u₃
inst✝² : Category.{v₃, u₃} E
inst✝¹ : MonoidalCategory E
inst✝ : BraidedCategory E
X₁ X₂ Y₁ Y₂ Z₁ Z₂ : C
⊢ ((α_ X₁ X₂ (Y₁ ⊗ Y₂)).hom ⊗ 𝟙 (Z₁ ⊗ Z₂)) ≫
      ((𝟙 X₁ ⊗ (α_ X₂ Y₁ Y₂).inv) ⊗ 𝟙 (Z₁ ⊗ Z₂)) ≫
        (α_ (X₁ ⊗ (X₂ ⊗ Y₁) ⊗ Y₂) Z₁ Z₂).inv ≫
          ((α_ X₁ ((X₂ ⊗ Y₁) ⊗ Y₂) Z₁).hom ⊗ 𝟙 Z₂) ≫
            ((𝟙 X₁ ⊗ (α_ (X₂ ⊗ Y₁) Y₂ Z₁).hom) ⊗ 𝟙 Z₂) ≫
              ((((((((𝟙 X₁ ⊗ (𝟙 X₂ ⊗ 𝟙 Y₁) ⊗ (β_ Y₂ Z₁).hom) ⊗ 𝟙 Z₂) ≫
                            ((𝟙 X₁ ⊗ (α_ (X₂ ⊗ Y₁) Z₁ Y₂).inv) ⊗ 𝟙 Z₂) ≫
                              ((𝟙 X₁ ⊗ (α_ X₂ Y₁ Z₁).hom ⊗ 𝟙 Y₂) ⊗ 𝟙 Z₂) ≫
                                ((𝟙 X₁ ⊗ (β_ X₂ (Y₁ ⊗ Z₁)).hom ⊗ 𝟙 Y₂) ⊗ 𝟙 Z₂) ≫
                                  ((𝟙 X₁ ⊗ (α_ Y₁ Z₁ X₂).hom ⊗ 𝟙 Y₂) ⊗ 𝟙 Z₂) ≫
                                    ((𝟙 X₁ ⊗ (α_ Y₁ (Z₁ ⊗ X₂) Y₂).hom) ⊗ 𝟙 Z₂)) ≫
                          ((𝟙 X₁ ⊗ (α_ Y₁ (Z₁ ⊗ X₂) Y₂).inv) ⊗ 𝟙 Z₂)) ≫
                        ((𝟙 X₁ ⊗ (α_ Y₁ Z₁ X₂).inv ⊗ 𝟙 Y₂) ⊗ 𝟙 Z₂)) ≫
                      (α_ X₁ (((Y₁ ⊗ Z₁) ⊗ X₂) ⊗ Y₂) Z₂).hom) ≫
                    (𝟙 X₁ ⊗ (α_ ((Y₁ ⊗ Z₁) ⊗ X₂) Y₂ Z₂).hom)) ≫
                  (𝟙 X₁ ⊗ (α_ (Y₁ ⊗ Z₁) X₂ (Y₂ ⊗ Z₂)).hom)) ≫
                (α_ X₁ (Y₁ ⊗ Z₁) (X₂ ⊗ Y₂ ⊗ Z₂)).inv =
    (α_ (X₁ ⊗ X₂) (Y₁ ⊗ Y₂) (Z₁ ⊗ Z₂)).hom ≫
      (𝟙 (X₁ ⊗ X₂) ⊗ tensor_μ C (Y₁, Y₂) (Z₁, Z₂)) ≫ tensor_μ C (X₁, X₂) (Y₁ ⊗ Z₁, Y₂ ⊗ Z₂)

Next tactic:
slice_lhs 11 12 =>
    rw [← tensor_comp, ← tensor_comp, Iso.hom_inv_id]
    simp