Essex' profile is surprisingly straight forward for a man as skilled in manipulation as he is. If he was looking for a plant, he'd most likely target someone who didn't survive our transition to this universe that he still had the genetic materials for, eliminate this world's version, and set up for us to discover and bring them in.

But it is also important to remember that Essex doesn't see the mansion as an enemy. Our clashes with him have always been tangential to whatever his specific goal is. So while he's happy to manipulate us, opportunistically steal resources like Collins, or use us as needed to help some aspect of his goals, unlike the Brotherhood or others, us all dead and the mansion in smoking ruins has never been one of his goals. That's not to say he wouldn't do it to achieve other ends, but it would be a radical departure from his past behaviours to target us that way.