Yhyjyk

The square-cube law holds true only for objects that are similar. In evolution, you can't make big leaps forward, but since most fantasy worlds are created by gods or people who think they're gods, I'm free to abuse Intelligent design.

Dragons have six limbs, the 2 wings are situated near the front legs, but just far enough not to interfere. Their anatomy as of now is pretty much the same as avian wings and flight muscles.

That being said, assuming dragon bones are much stronger, thanks to some nanoscale engineering and a hint of graphene, how could the pectoralis major, and the bone connecting to it, be rearranged to produce more power for the same mass?

Note: Before we veer off into the deepest insanity, I was thinking more of the "If strength is the function of muscle cross-section, can't we just shorten the fibers and increase the cross-sectional area, like a boss?" path.

When I was a teen it was common for us to have a 50 cc scooter which, by law, could not exceed the speed of 50 km/h. When some of us wanted to tune up the scooter and get more out of the engine, one of the trick was to change the carburetor or the exhaust (or both). (don't try this at home, going at 110 km/h on normal roads with something designed to go at 50 is not only illegal, but also mighty stupid and a fast way to have an early funeral)

This trick would have allowed the engine to output more power with the same volume of the cylinder and the same structure.

How does this apply to your dragons? Well, you don't need to redesign the muscles/engine, just increase the metabolism of the beast, allowing it to burn more nutrients and output more energy with the same structure.

Incidentally, this is the same trick used by birds, which allow them to be able to fly.

Your dragons are currently big pigeons: giant pectoralis for downstroke, presumably proportionately small supercoracoideus for upstroke. Background reading

But you could take advantage of graphene and the rule of cool and model your dragons on a unique bird: the hummingbird.

https://en.wikipedia.org/wiki/Bird_flight

Most birds that hover have high aspect ratio wings that are suited to
low speed flying. Hummingbirds are a unique exception – the most
accomplished hoverers of all birds. Hummingbird flight is different
from other bird flight in that the wing is extended throughout the
whole stroke, which is a symmetrical figure of eight, with the wing
producing lift on both the up- and down-stroke. Hummingbirds beat
their wings at some 43 times per second, while others may be as high
as 80 times per second.

Hummingbirds fly like insects. Their wings move with a sort of sculling motion, and the pectoralis and supercoracoideus are closer to symmetrical in their contributions. They generate vortices as part of their lift mechanism, which would be so cool for a dragon because it would generate dust devils close to the ground.

One could argue dragons are too big and heavy to fly like hummingbirds. I refer these naysayers above to "graphene and rule of cool" and suggest they devote their skeptical energies to the problems inherent in breathing fire.

how could the pectoralis major, and the bone connecting to it, be rearranged to produce more power for the same mass?

The pectoralis major connects to the arm bone, so I am going to offer a solution that is biologically plausible:

Due to some mutation, some dragons are born with thw front legs and the wings partially fused. This adds a lot of muscle power to each wing stroke.

Over millenia (or maybe longer spans), the dragons evolve to have only four limbs. The wings get ever more muscular, achieving your desired result.