If he was going to live there for a long time, I'd agree.
He's vindictive enough that he would want to observe her grave and relish it. Like under a newly poured concrete slab or patio, or under grove of newly planted trees.
But he actually desperately need to sell a couple of those properties, so he would have to be very selective.
On the other hand, essentially destroying her mortal remains would give him some sort of perverted mental triumph. Obliterating her from the face of the earth.
I think that trying to dispose of a dismembered adult human body by putting it into bags that you put in the dumpster isn't really enough for him.
I'd have to go back to what tanner's do (his father was a tanner and he worked with him ). Like a ritualistic killing with rendering of his tissues.
I believe the disposal of her body, her remains, all traces of her is deeply personal and ritualistic to him. Digging a grave and pouring concrete over the top isn't enough. Putting her in a river anchored by concrete isn't enough.
This act really means something to him because it's his revenge on her and all her family. Revenge on her parents. It's something we haven't even got to yet
So I can come up with lots of reasons why he would be extra-careful about not putting her near any property that he would lose access to.