So, I have devoted an unhealthy amount of my time to the dragon question, and a final solution will have to wait... for several thousand years at least.
My original plan was to make them at around 180 cm tall at the shoulders, and 6.4 meters long, 2.8 meters of which is the tail.
They soar for most of the time, and can't quad launch, so they need an elevated point to take off, and use their flight muscles, typically for about 90 seconds, to gain altitude.
Flight muscles can't be optimized any further, though you can always attach more. Problem is, you need to reinforce everything else to be able to withstand the increased load. My idea is to use:
For the wing membrane and tendons. Spider Silk can have tensile strengths of 1,6 GPa, sure, supercotraction can potentially ruin it, but it is still too damn strong. You might wanna braid them, though. The elastic modulus is still kind of a problem.
At least their structure. Limpet tooth is nature's answer to short-fiber composites and can be equated to them. Bones could be structured this way. Tensile strength is 4.9 GPa, yay.
The question is if this reinforcing could work as intended. I have faith in goethite, but I'm unsure about the other. I'm mainly concerned about the wing. Would these reinforcements allow me to make it significantly larger?
Storywise, dragons were designed creatures, and their creator spent several thousand years, according to some sources, creating them.
Enhancements are both internal (wing bones, ribcage, tendons, ligaments) and external (wing membrane, and its attachment).
For the sake of simplicity, and my own sanity, we're going to assume that the wing structure is similar to that of the Quetzalcoatlus northropi.