Yup - six inches is fine for height - even enough room for them to hang out on top of their hide.
Worst case scenario, you try the tub and decide you don't like it and can go back to the tank. But I think after a few weeks in the tub, you'll find that keeping her husbandry correct and cleaning will be so much easier, and she'll probably be happier too!
The one you have is fine - as long as you properly secure it so that she can't escape it - I don't trust that type of lid. If you had a rack you could slide it into without the lid - even better!![]()