Yes its much more dense and heavier.
I don't rate cumulus at all especially for the stairs. In my experience it softens too quick.
I'd recommend conn 8mm for stairs but the conn 10 is ideal as well
Take up the ceramics and ply out with 9mm if you want to fit the lvt on top.
If floor height isn't a concern then providing your existing tiles are solid then you could flow over it with a fibre based screed.