Let's start with a real-life animal that's roughly where we want dragons to be. Quetzalcoatlus is all very well, but I prefer my dragons a bit more robust. Say we use Haast's eagle, a very big extinct eagle that hunted moa: Weight up to 30 pounds, body length about 4 feet, wingspan up to 10 feet, able to take off from the ground.
How big should dragons be? Let's figure the tail is extremely long and thin, more of a steering vane. For the body, say 40 feet. That's on par with the very largest minis (8x8 base). So, this dragon is 10 times the size of Haast's eagle in every dimension. Thus, it must be 10 times as efficient a flier.
You put this as a footnote. It ought to be up front:
If the dragon's body is 10 times the size of Haast's eagle
in every dimension, that means it is 1000 times the volume. To first approximation, that means it is 1000 times the mass and weight.
Now, here's where you run into problems. The lift generated by a wing is directly proportional to the wing's
surface area. But, at 10 times the size in each dimension, the surface area of this dragon's wings are only 100 times those of the eagle.
Increase the size - you get 1000 times the weight, but only 100 times the lift. That's the problem.
You said you preferred body plans like the eagle. Well, this is why Quetzalcoatlus didn't have the eagle's body plan - keep the same body plan at a larger size, and you can't fly - because for natural animals, you have only a few options for reducing the body mass, and most of those actually negate the point of being physically larger!
This is where being a fantasy creature comes in. I am not sure most of the time that you want them to be 1000 times the physical size, but only 100 times the weight. In our case, 100 times the weight of the eagle is only 3000 pounds - on the order of the weight of a large bull. Aside from blowing away in the wind... it would mean that any predator who could take down a large bull might also have a chance at the dragon.
So, no, let us not reduce the weight. Instead, increase how much you get for that weight. In normal animals, as you say, the force muscles can exert is proportional to the cross sectional area of the muscle fiber bundle - if I am 10 times the size in each dimension, I am 1000 times the weight, but only 100 times as strong. Whoops!
Fix that one thing - muscle power, and it all works. The muscles of our large fantasy creatures are incredibly powerful as compared to our smaller fantasy creatures. Giants, Dragons, and so on, have some force multiplier. Their connective tissues and bone strength (not overall weight, just their strength) must increase to take the larger loads. But then, you're done.
So, really powerful muscle, and dragon and giant bones that are stronger than metals. And that's cool anyway.