Reading your thesis is an essential viva prep activity, but it’s not for the purpose of memorising a book. You don’t have to be ready to recall everything you’ve ever done or written in your viva.
You read your thesis after submission to get a good sense of the flow of information you’ve laid out. You read your thesis to have a feel for where you can find key sections and ideas, not to imprint them in your brain so that your thesis is redundant. At your viva you can use your thesis: it’s there as a resource!
Before your viva, read your thesis to get a good feel for the flow of ideas. At your viva, use what you know and remember to respond to questions.