Think I've cracked this.

I came here as I was trying:

  ECSService:
    Type: AWS::ECS::Service
    DependsOn:
      Fn::If:
        - SomeConditionIsTrue
        - [ AnotherECSService, OneMoreECSService ]
        - AnotherECSService
    Properties:

And AWS::Transform was failing

But this seems to have worked:

  ECSService:
    Type: AWS::ECS::Service
    Fn::If:
      - SomeConditionIsTrue
      - DependsOn: [ AnotherECSService, OneMoreECSService ]
      - DependsOn: AnotherECSService
    Properties:

Luke