Well my first guess is to use the two large flyers we know of: birds and pterosaurs. Luckily for us they have legs-under-wings and legs-behind-wings respectively.
Hind limbs should probably shoot straight back to minimise drag, as with pterosaurs and bats (though they also contribute to flightsurface area with their membranes). Consider making those hind limbs smaller/less muscular to suit this, as when landing all that weight in the chest will need to be carried by the forelimbs (probably not the small legs and more likely the wings them selfs like pterosaurs which also might mean hexapodal liftoff). If you don’t like the idea of splayed out legs, then look to vultures and eagles for inspiration for leg posture.
Forelimbs (arms?) will likely tuck under like a bird, though since they seem to sit a little forward from the wing shoulders maybe they would also sling back. Either way, the main goal is to reduce drag, so whichever posture would streamline the body the most is the posture that nature would promote.
As for neck posture, no ‘S’ poses while in flight unless they are capable of really folding it up - you’ll notice this in herons and stalks etc, although they have more of a ‘5’ than an ‘S’. basically the more plane-like your dragon looks, the easier time it’ll have flying.
Not the most agile creatures, herons. Also those legs might be another example of realistic hind leg posture.
The major difference in birds when compared to the scaly flyers is they can somewhat make up for a non-aerodynamic body shape with the volume of their feathers - i.e tucking their legs into down. Take this into account.
TLDR: for neck posture, straight-necked or heron-necked.
For forelegs, look at small birds (tucked under) or eagles and vultures (slung back) depending on what they use them for.
For hind legs, think pterosaur or bat but vulture/eagle isn’t completely out of the question.