set_option maxHeartbeats 400000 in
theorem associator_monoidal (X₁ X₂ X₃ Y₁ Y₂ Y₃ : C) :
    tensor_μ C (X₁ ⊗ X₂, X₃) (Y₁ ⊗ Y₂, Y₃) ≫
        (tensor_μ C (X₁, X₂) (Y₁, Y₂) ⊗ 𝟙 (X₃ ⊗ Y₃)) ≫ (α_ (X₁ ⊗ Y₁) (X₂ ⊗ Y₂) (X₃ ⊗ Y₃)).hom =
      ((α_ X₁ X₂ X₃).hom ⊗ (α_ Y₁ Y₂ Y₃).hom) ≫
        tensor_μ C (X₁, X₂ ⊗ X₃) (Y₁, Y₂ ⊗ Y₃) ≫ (𝟙 (X₁ ⊗ Y₁) ⊗ tensor_μ C (X₂, X₃) (Y₂, Y₃)) := by
  have :
    (α_ (X₁ ⊗ Y₁) (X₂ ⊗ Y₂) (X₃ ⊗ Y₃)).hom =
      ((α_ X₁ Y₁ (X₂ ⊗ Y₂)).hom ⊗ 𝟙 (X₃ ⊗ Y₃)) ≫
        ((𝟙 X₁ ⊗ (α_ Y₁ X₂ Y₂).inv) ⊗ 𝟙 (X₃ ⊗ Y₃)) ≫
          (α_ (X₁ ⊗ (Y₁ ⊗ X₂) ⊗ Y₂) X₃ Y₃).inv ≫
            ((α_ X₁ ((Y₁ ⊗ X₂) ⊗ Y₂) X₃).hom ⊗ 𝟙 Y₃) ≫
              ((𝟙 X₁ ⊗ (α_ (Y₁ ⊗ X₂) Y₂ X₃).hom) ⊗ 𝟙 Y₃) ≫
                (α_ X₁ ((Y₁ ⊗ X₂) ⊗ Y₂ ⊗ X₃) Y₃).hom ≫
                  (𝟙 X₁ ⊗ (α_ (Y₁ ⊗ X₂) (Y₂ ⊗ X₃) Y₃).hom) ≫
                    (𝟙 X₁ ⊗ (α_ Y₁ X₂ ((Y₂ ⊗ X₃) ⊗ Y₃)).hom) ≫
                      (α_ X₁ Y₁ (X₂ ⊗ (Y₂ ⊗ X₃) ⊗ Y₃)).inv ≫
                        (𝟙 (X₁ ⊗ Y₁) ⊗ 𝟙 X₂ ⊗ (α_ Y₂ X₃ Y₃).hom) ≫
                          (𝟙 (X₁ ⊗ Y₁) ⊗ (α_ X₂ Y₂ (X₃ ⊗ Y₃)).inv) :=
    by pure_coherence
  rw [this]; clear this
  slice_lhs 2 4 => rw [← tensor_comp, ← tensor_comp, tensor_μ_def₁, tensor_comp, tensor_comp]
  slice_lhs 4 5 => rw [← tensor_id, associator_inv_naturality]
  slice_lhs 5 6 => rw [← tensor_comp, associator_naturality, tensor_comp]
  slice_lhs 6 7 =>
    rw [← tensor_comp, ← tensor_comp, associator_naturality, tensor_comp, tensor_comp]
  have :
    ((α_ X₁ X₂ (Y₁ ⊗ Y₂)).hom ⊗ 𝟙 (X₃ ⊗ Y₃)) ≫
        ((𝟙 X₁ ⊗ (α_ X₂ Y₁ Y₂).inv) ⊗ 𝟙 (X₃ ⊗ Y₃)) ≫
          (α_ (X₁ ⊗ (X₂ ⊗ Y₁) ⊗ Y₂) X₃ Y₃).inv ≫
            ((α_ X₁ ((X₂ ⊗ Y₁) ⊗ Y₂) X₃).hom ⊗ 𝟙 Y₃) ≫ ((𝟙 X₁ ⊗ (α_ (X₂ ⊗ Y₁) Y₂ X₃).hom) ⊗ 𝟙 Y₃) =
      (α_ (X₁ ⊗ X₂) (Y₁ ⊗ Y₂) (X₃ ⊗ Y₃)).hom ≫
        (𝟙 (X₁ ⊗ X₂) ⊗ (α_ (Y₁ ⊗ Y₂) X₃ Y₃).inv) ≫
          (α_ X₁ X₂ (((Y₁ ⊗ Y₂) ⊗ X₃) ⊗ Y₃)).hom ≫
            (𝟙 X₁ ⊗ (α_ X₂ ((Y₁ ⊗ Y₂) ⊗ X₃) Y₃).inv) ≫
              (α_ X₁ (X₂ ⊗ (Y₁ ⊗ Y₂) ⊗ X₃) Y₃).inv ≫
                ((𝟙 X₁ ⊗ 𝟙 X₂ ⊗ (α_ Y₁ Y₂ X₃).hom) ⊗ 𝟙 Y₃) ≫
                  ((𝟙 X₁ ⊗ (α_ X₂ Y₁ (Y₂ ⊗ X₃)).inv) ⊗ 𝟙 Y₃) :=
    by pure_coherence
  slice_lhs 2 6 => rw [this]
  clear this
  slice_lhs 1 3 => rw [tensor_μ_def₁]
  slice_lhs 3 4 => rw [← tensor_id, associator_naturality]
  slice_lhs 4 5 => rw [← tensor_comp, associator_inv_naturality, tensor_comp]
  slice_lhs 5 6 => rw [associator_inv_naturality]
  slice_lhs 6 9 =>
    rw [← tensor_comp, ← tensor_comp, ← tensor_comp, ← tensor_comp, ← tensor_comp, ← tensor_comp,
      tensor_id, associator_monoidal_aux, ← id_comp (𝟙 X₁ ≫ 𝟙 X₁ ≫ 𝟙 X₁ ≫ 𝟙 X₁), ←
      id_comp (𝟙 X₁ ≫ 𝟙 X₁ ≫ 𝟙 X₁ ≫ 𝟙 X₁ ≫ 𝟙 X₁), ← id_comp (𝟙 Y₃ ≫ 𝟙 Y₃ ≫ 𝟙 Y₃ ≫ 𝟙 Y₃), ←
      id_comp (𝟙 Y₃ ≫ 𝟙 Y₃ ≫ 𝟙 Y₃ ≫ 𝟙 Y₃ ≫ 𝟙 Y₃), 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 X₃ Y₁ Y₂ Y₃ : C
⊢ (α_ (X₁ ⊗ X₂) X₃ ((Y₁ ⊗ Y₂) ⊗ Y₃)).hom ≫
      (𝟙 (X₁ ⊗ X₂) ⊗ (α_ X₃ (Y₁ ⊗ Y₂) Y₃).inv) ≫
        (α_ X₁ X₂ ((X₃ ⊗ Y₁ ⊗ Y₂) ⊗ Y₃)).hom ≫
          (𝟙 X₁ ⊗ (α_ X₂ (X₃ ⊗ Y₁ ⊗ Y₂) Y₃).inv) ≫
            (α_ X₁ (X₂ ⊗ X₃ ⊗ Y₁ ⊗ Y₂) Y₃).inv ≫
              ((((((((𝟙 X₁ ⊗ (α_ X₂ X₃ (Y₁ ⊗ Y₂)).inv) ⊗ 𝟙 Y₃) ≫
                            ((𝟙 X₁ ⊗ (α_ (X₂ ⊗ X₃) Y₁ Y₂).inv) ⊗ 𝟙 Y₃) ≫
                              ((𝟙 X₁ ⊗ (β_ (X₂ ⊗ X₃) Y₁).hom ⊗ 𝟙 Y₂) ⊗ 𝟙 Y₃) ≫
                                ((𝟙 X₁ ⊗ (α_ Y₁ X₂ X₃).inv ⊗ 𝟙 Y₂) ⊗ 𝟙 Y₃) ≫
                                  ((𝟙 X₁ ⊗ (α_ (Y₁ ⊗ X₂) X₃ Y₂).hom) ⊗ 𝟙 Y₃) ≫
                                    ((𝟙 X₁ ⊗ 𝟙 (Y₁ ⊗ X₂) ⊗ (β_ X₃ Y₂).hom) ⊗ 𝟙 Y₃)) ≫
                          (α_ X₁ ((Y₁ ⊗ X₂) ⊗ Y₂ ⊗ X₃) Y₃).hom) ≫
                        (𝟙 X₁ ⊗ (α_ (Y₁ ⊗ X₂) (Y₂ ⊗ X₃) Y₃).hom)) ≫
                      (𝟙 X₁ ⊗ (α_ Y₁ X₂ ((Y₂ ⊗ X₃) ⊗ Y₃)).hom)) ≫
                    (α_ X₁ Y₁ (X₂ ⊗ (Y₂ ⊗ X₃) ⊗ Y₃)).inv) ≫
                  (𝟙 (X₁ ⊗ Y₁) ⊗ 𝟙 X₂ ⊗ (α_ Y₂ X₃ Y₃).hom)) ≫
                (𝟙 (X₁ ⊗ Y₁) ⊗ (α_ X₂ Y₂ (X₃ ⊗ Y₃)).inv) =
    ((α_ X₁ X₂ X₃).hom ⊗ (α_ Y₁ Y₂ Y₃).hom) ≫
      tensor_μ C (X₁, X₂ ⊗ X₃) (Y₁, Y₂ ⊗ Y₃) ≫ (𝟙 (X₁ ⊗ Y₁) ⊗ tensor_μ C (X₂, X₃) (Y₂, Y₃))

Next tactic:
slice_lhs 11 12 => rw [associator_naturality]