-- Mathlib/CategoryTheory/Monoidal/Bimod.lean
theorem hom_right_act_hom' :
    ((P.tensorBimod Q).tensorBimod L).actRight ≫ hom P Q L =
      (hom P Q L ▷ U.X) ≫ (P.tensorBimod (Q.tensorBimod L)).actRight

Proof state:
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
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_ (P.tensorBimod Q).X L.X U.X).hom ≫
      (P.tensorBimod Q).X ◁ L.actRight ≫
        (PreservesCoequalizer.iso (tensorRight L.X) (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft)).inv ≫
          coequalizer.desc
            ((α_ P.X Q.X L.X).hom ≫
              P.X ◁ coequalizer.π (Q.actRight ▷ L.X) ((α_ Q.X T.X L.X).hom ≫ Q.X ◁ L.actLeft) ≫
                coequalizer.π (P.actRight ▷ TensorBimod.X Q L)
                  ((α_ P.X S.X (TensorBimod.X Q L)).hom ≫ P.X ◁ TensorBimod.actLeft Q L))
            ⋯ =
    coequalizer.π ((P.tensorBimod Q).actRight ▷ L.X)
          ((α_ (P.tensorBimod Q).X T.X L.X).hom ≫ (P.tensorBimod Q).X ◁ L.actLeft) ▷
        U.X ≫
      coequalizer.desc
            ((PreservesCoequalizer.iso (tensorRight L.X) (P.actRight ▷ Q.X)
                  ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft)).inv ≫
              coequalizer.desc
                ((α_ P.X Q.X L.X).hom ≫
                  P.X ◁ coequalizer.π (Q.actRight ▷ L.X) ((α_ Q.X T.X L.X).hom ≫ Q.X ◁ L.actLeft) ≫
                    coequalizer.π (P.actRight ▷ TensorBimod.X Q L)
                      ((α_ P.X S.X (TensorBimod.X Q L)).hom ≫ P.X ◁ TensorBimod.actLeft Q L))
                ⋯)
            ⋯ ▷
          U.X ≫
        TensorBimod.actRight P (Q.tensorBimod L)

Tactic:
slice_rhs 1 2 => rw [← comp_whiskerRight, coequalizer.π_desc, comp_whiskerRight]

Result:
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
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_ (P.tensorBimod Q).X L.X U.X).hom ≫
      (P.tensorBimod Q).X ◁ L.actRight ≫
        (PreservesCoequalizer.iso (tensorRight L.X) (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft)).inv ≫
          coequalizer.desc
            ((α_ P.X Q.X L.X).hom ≫
              P.X ◁ coequalizer.π (Q.actRight ▷ L.X) ((α_ Q.X T.X L.X).hom ≫ Q.X ◁ L.actLeft) ≫
                coequalizer.π (P.actRight ▷ TensorBimod.X Q L)
                  ((α_ P.X S.X (TensorBimod.X Q L)).hom ≫ P.X ◁ TensorBimod.actLeft Q L))
            ⋯ =
    ((PreservesCoequalizer.iso (tensorRight L.X) (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft)).inv ▷
          U.X ≫
        coequalizer.desc
            ((α_ P.X Q.X L.X).hom ≫
              P.X ◁ coequalizer.π (Q.actRight ▷ L.X) ((α_ Q.X T.X L.X).hom ≫ Q.X ◁ L.actLeft) ≫
                coequalizer.π (P.actRight ▷ TensorBimod.X Q L)
                  ((α_ P.X S.X (TensorBimod.X Q L)).hom ≫ P.X ◁ TensorBimod.actLeft Q L))
            ⋯ ▷
          U.X) ≫
      TensorBimod.actRight P (Q.tensorBimod L)

Proof state:
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
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_ (P.tensorBimod Q).X L.X U.X).hom ≫
      (P.tensorBimod Q).X ◁ L.actRight ≫
        (PreservesCoequalizer.iso (tensorRight L.X) (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft)).inv ≫
          coequalizer.desc
            ((α_ P.X Q.X L.X).hom ≫
              P.X ◁ coequalizer.π (Q.actRight ▷ L.X) ((α_ Q.X T.X L.X).hom ≫ Q.X ◁ L.actLeft) ≫
                coequalizer.π (P.actRight ▷ TensorBimod.X Q L)
                  ((α_ P.X S.X (TensorBimod.X Q L)).hom ≫ P.X ◁ TensorBimod.actLeft Q L))
            ⋯ =
    ((PreservesCoequalizer.iso (tensorRight L.X) (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft)).inv ▷
          U.X ≫
        coequalizer.desc
            ((α_ P.X Q.X L.X).hom ≫
              P.X ◁ coequalizer.π (Q.actRight ▷ L.X) ((α_ Q.X T.X L.X).hom ≫ Q.X ◁ L.actLeft) ≫
                coequalizer.π (P.actRight ▷ TensorBimod.X Q L)
                  ((α_ P.X S.X (TensorBimod.X Q L)).hom ≫ P.X ◁ TensorBimod.actLeft Q L))
            ⋯ ▷
          U.X) ≫
      TensorBimod.actRight P (Q.tensorBimod L)

Tactic:
refine (cancel_epi ((tensorRight _ ⋙ tensorRight _).map (coequalizer.π _ _))).1 ?_

Result:
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
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
⊢ (tensorRight L.X ⋙ tensorRight U.X).map (coequalizer.π (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft)) ≫
      (α_ (P.tensorBimod Q).X L.X U.X).hom ≫
        (P.tensorBimod Q).X ◁ L.actRight ≫
          (PreservesCoequalizer.iso (tensorRight L.X) (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft)).inv ≫
            coequalizer.desc
              ((α_ P.X Q.X L.X).hom ≫
                P.X ◁ coequalizer.π (Q.actRight ▷ L.X) ((α_ Q.X T.X L.X).hom ≫ Q.X ◁ L.actLeft) ≫
                  coequalizer.π (P.actRight ▷ TensorBimod.X Q L)
                    ((α_ P.X S.X (TensorBimod.X Q L)).hom ≫ P.X ◁ TensorBimod.actLeft Q L))
              ⋯ =
    (tensorRight L.X ⋙ tensorRight U.X).map
        (coequalizer.π (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft)) ≫
      ((PreservesCoequalizer.iso (tensorRight L.X) (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft)).inv ▷
            U.X ≫
          coequalizer.desc
              ((α_ P.X Q.X L.X).hom ≫
                P.X ◁ coequalizer.π (Q.actRight ▷ L.X) ((α_ Q.X T.X L.X).hom ≫ Q.X ◁ L.actLeft) ≫
                  coequalizer.π (P.actRight ▷ TensorBimod.X Q L)
                    ((α_ P.X S.X (TensorBimod.X Q L)).hom ≫ P.X ◁ TensorBimod.actLeft Q L))
              ⋯ ▷
            U.X) ≫
        TensorBimod.actRight P (Q.tensorBimod L)