First:
Staging doesn't seem to work? The rear heatshield appears to tie the shuttle to the separator, and the only way to separate is by burning off the first stage?
That has to be fixed.
There's two ways, one is you can use a docking port instead of a separator, or... use an angled interstage like so:
View attachment 103320
That way it doesn't connect to anything.
Second:
I was able to take 30t of fuel away from the shuttle and 50t of fuel away from the booster and
it still worked without making the payload lighter. Try dothat, so it becomes more efficient in terms of payload to launch mass ratio.
Hint: you don't need so much fuel for the door.
Third:
When you make it lighter, you don't need three titans on the booster. Two titans is enough. This makes it even lighter, upward positive cycle.
Fourth:
If you separate a nose tank from the rest of the shuttle, you can dump unnecessary fuel into the nose of the shuttle, so that when you're trying to re-enter, it won't try to spin the engines into the flames even with RCS off. Small balancing fix, like trim tanks on planes and submarines.
Fifth:
Since you're bp editing anyway, fix the nose heatshields so that they look better.
Not required to fix, but nice if you do:
It's best if you can access everything without staging. It would be a good idea to attach the payload docking port to the door instead of the shuttle, so you can take out the door and then easily detach the payload without staging.
Also the door doesn't need a docking port at each end, one end is enough. At the other end you can leave a small gap between the door and the rest of the shuttle, that gives you space to easily dock it back.
Don't hesitate to ask for clarification on my points, and if you need piloting tips to fly with less fuel just ask.