theorem hom_left_act_hom' :
    ((P.tensorBimod Q).tensorBimod L).actLeft ≫ hom P Q L =
      (𝟙 R.X ⊗ hom P Q L) ≫ (P.tensorBimod (Q.tensorBimod L)).actLeft := by
  dsimp; dsimp [hom, homAux]
  refine' (cancel_epi ((tensorLeft _).map (coequalizer.π _ _))).1 _
  rw [tensorLeft_map]
  slice_lhs 1 2 => rw [TensorBimod.id_tensor_π_actLeft]
  slice_lhs 3 4 => rw [coequalizer.π_desc]
  slice_rhs 1 2 => rw [← id_tensor_comp, coequalizer.π_desc, id_tensor_comp]
  refine' (cancel_epi ((tensorRight _ ⋙ tensorLeft _).map (coequalizer.π _ _))).1 _
  dsimp; dsimp [TensorBimod.X]
  slice_lhs 1 2 => rw [associator_inv_naturality]
  slice_lhs 2 3 =>
    rw [← comp_tensor_id, TensorBimod.id_tensor_π_actLeft, comp_tensor_id, comp_tensor_id]
  slice_lhs 4 6 => rw [π_tensor_id_preserves_coequalizer_inv_desc]
  slice_lhs 3 4 => rw [associator_naturality]
  slice_lhs 4 5 => 

Proof state:
case a.a.a.a
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
A B : Mon_ C
M : Bimod A B
inst✝² : HasCoequalizers C
inst✝¹ : (X : C) → PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorLeft X)
inst✝ : (X : C) → PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorRight X)
R S T U : Mon_ C
P : Bimod R S
Q : Bimod S T
L : Bimod T U
| (P.actLeft ⊗ 𝟙 Q.X ⊗ 𝟙 L.X) ≫
    (𝟙 P.X ⊗ coequalizer.π (Q.actRight ⊗ 𝟙 L.X) ((α_ Q.X T.X L.X).hom ≫ (𝟙 Q.X ⊗ L.actLeft)))
case a.a.a.a
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
A B : Mon_ C
M : Bimod A B
inst✝² : HasCoequalizers C
inst✝¹ : (X : C) → PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorLeft X)
inst✝ : (X : C) → PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorRight X)
R S T U : Mon_ C
P : Bimod R S
Q : Bimod S T
L : Bimod T U
| coequalizer.π (P.actRight ⊗ 𝟙 (coequalizer (Q.actRight ⊗ 𝟙 L.X) ((α_ Q.X T.X L.X).hom ≫ (𝟙 Q.X ⊗ L.actLeft))))
    ((α_ P.X S.X (coequalizer (Q.actRight ⊗ 𝟙 L.X) ((α_ Q.X T.X L.X).hom ≫ (𝟙 Q.X ⊗ L.actLeft)))).hom ≫
      (𝟙 P.X ⊗ TensorBimod.actLeft Q L))
case a
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
A B : Mon_ C
M : Bimod A B
inst✝² : HasCoequalizers C
inst✝¹ : (X : C) → PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorLeft X)
inst✝ : (X : C) → PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorRight X)
R S T U : Mon_ C
P : Bimod R S
Q : Bimod S T
L : Bimod T U
| (α_ R.X (P.X ⊗ Q.X) L.X).inv
case a.a
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
A B : Mon_ C
M : Bimod A B
inst✝² : HasCoequalizers C
inst✝¹ : (X : C) → PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorLeft X)
inst✝ : (X : C) → PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorRight X)
R S T U : Mon_ C
P : Bimod R S
Q : Bimod S T
L : Bimod T U
| (α_ R.X P.X Q.X).inv ⊗ 𝟙 L.X
case a.a.a
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
A B : Mon_ C
M : Bimod A B
inst✝² : HasCoequalizers C
inst✝¹ : (X : C) → PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorLeft X)
inst✝ : (X : C) → PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorRight X)
R S T U : Mon_ C
P : Bimod R S
Q : Bimod S T
L : Bimod T U
| (α_ (R.X ⊗ P.X) Q.X L.X).hom

Next tactic:
rw [MonoidalCategory.tensor_id, tensor_id_comp_id_tensor]