I have an idea that's a little depressing and has some moral quandary. Basically a reversal of your Idea #3

Maybe after living so long, it doesn't want to get tied to the mortal realm by having worshippers. I don't know how proven the afterlife is in your setting, but the dragon may well want to be rid of its current existence and prefer annihilation or transcending to another plane, as opposed to being the pseudo-deity of these kobolds and the natural environment it's already been stuck at so long it's tired of things.

It wants the PCs to eliminate its worshippers and knowledge of it before it's too late. I'm assuming the "kobolds" aren't overly evil and eliminating them wouldn't be something normal to do. The dragon might lie (or try to lie, but do so poorly due to its limited interaction with mortals who don't revere it as a god) about them to motivate the PCs, but in the end it's a powerful and fearful dragon asking the PCs to eliminate a monstrous race that doesn't seem to be doing anything bad and actually likes the dragon.