you've got an impressive amount of studio equipment there. Okay, maybe it's not what the big studios out there would use but it's always good enough. That said, I can't quite understand why you want to buy an Apollo Interface.
In my humble opinion your next steps should be: Room treatment. This can easily be the amount of 2 to 4 time what the Apollo costs. But it's more important. Depending on the room, see that you have the right floor, ceiling, bass traps and don't put napped foam on the walls. 😉
You could think about better monitors, but if you are used to the HS8 stick with them, since they are very good in that price range.