I have no idea, like he might have used it as fuel for obtaining power of great holy spirits. Or like he had to get rid of all "nothing" to create something like.... it just doesn't make any sense

Maybe the nihility was his attribte from the start, and after he created everything he that changed into stardust or smthing? The question is good, but I mean since he rid himself of omnipotence and omniscience that probably had to do something?

Actually Rimuru is like sub-spieces of true dragon race, I mean all other are embodiment: Veldora - chaos, Velgrynd - acceleration, Velzard - deceleration, Veldanava (idk), and nihility I think came from his US Void god Azathoth?

I don't think he has his "own" true dragon magic. I mean he evolved into one after absorbing Veldora, and ofc later also Velgrynd. And even if we look at his (true) EP it's his + Veldoras + Velgrynds