Read your thesis to refresh your memory, not commit it to photographic recall.
Make notes in the margin or highlight sections to make things stand out; don’t seek to find problems or more to add to your thesis.
Read recent papers or your examiners’ publications to build your competence, not to expand your bibliography.
Have a mock viva to rehearse for your own, not to predict or perfect your approach.
Preparation is not about perfection at all. It’s about finding confidence in what you’ve done already; being sure that you’ve already reached a point where you are good enough.
Lots of work, a little prep and you are ready for the viva.