Your dragon does not need to be "flashy" in terms of lightning power. In fact electricity often goes unnoticed except of course when an electric arc forms.

There are two paths to make this into a plausible natural weapon!

The first is a conductive liquid close in composition to battery liquid, which is often acidic. This liquid paired with an electric organ and bam! You AMP up the damage.

The second is static electricity. I don't know if you've heard about this but the arc of the covenant is extremely conductive! So much in fact that the researchers that tried to replicate it had to stop because of the danger it posed. This is mostly due to the materials it was made out of but also the wings of the angels on top touching each other. You can have your electric dragon join the tips of its wings and shock everything to death! You can also give it golden scales, pretty sweet right?

Your dragon needs not to be "flashy"

In fact electricity often goes unnoticed except of course when an electric arc forms. Also no creature can ever harness much less withstand a lightning strike.

Two solutions:

The first is a conductive liquid similar to battery liquid, which by the way is often acidic. When a lead acid battery is fully charged, the electrolyte is composed of a solution that consists of up to 40% sulfuric acid, with the remainder consisting of regular water. As the battery discharges, the positive and negative plates gradually turn into lead sulfate. The electrolyte loses much of its sulfuric acid content during this process, and it eventually becomes a very weak solution of sulfuric acid and water.

Since this is a reversible chemical process, charging a car battery causes the positive plates to turn back into lead oxide, while the negative plates turn back into pure, spongy lead, and the electrolyte becomes a stronger solution of sulfuric acid and water. This process can occur many thousands of times.

Your lighting dragon would simply have to possess an organ that stores electricity in this manner. You can also have your dragon spray its target with a conductive liquid and then zap them with an electric organ.

The second is static electricity. I don't know if you've heard about this but the arc of the covenant is extremely conductive! It is basically a giant capacitor. Mostly made of wood and gold the arc could effectively collect static electricity from the ground and the air. The two cherubim placed on top had their wings touch each other which functioned as the positive poll. Your dragons scales can be made similarly, wooden but coated with gold (I assume your dragon is a fan treasures). Then to unleash an electric attack it would simply join the tips of its wings together to generate a current.

Your are free to use both at the same time for a dangerous dragon. It would develop a need for both gold and sulfur to sustain this power however.