As long as you’re just using CSS you can’t actually move the completed tasks, but you can indeed hide them.
However the CSS selectors you’ve provided, i.e. .markdown-preview-view relates to the reading mode, and not the live preview variant. There it needs a different selector to target the completed tasks, and it most likely involves something with .HyperMD... or similar.
Hopefully some of the CSS gurus of this forum might pick up on this, as I’m not currently at a place nor do I’ve got the time where I can delve into looking for the correct CSS selectors for a completed tasks in live preview.