This whole conversation reminds moi of something from 40k, an overblown weapon (like all 40k weapons) called the life eater virus.
It's used for planetary extermination and is a supervirus that simply consumes organic matter and reproduces at an impossible rate. As all living organisms are broken down in minutes by it the decomposition releases vast amounts of oxygen and flammabel gasses, which makes the atmosphere highly flammable and creates global firestorms as the air itself burns.
https://wh40k.lexicanum.com/wiki/Virus_bomb
Yeah, it's 40k grimdark and completely impossible but this topic just reminded me of it so much and I guess it meets the OP issue for something that would wipe out all life but leave the planet intact.