So a thief of void ... would a thief of void eventually become intangible due to there abundance of nothingness? And would a thief of void steal away other people’s irrelevance but at the cost of making the thief irrelevant

They could definitely steal other’s irrelevance, leaving themselves less and less relevant, while bumping others, but also remember they could Steal Horrorterror Stuff and go all Grimdark, so that’s a possibility as well.