Why Wait?
Friendly advice time: take a break from your research when you submit your thesis.
Give yourself time to rest, plan your next steps and ask for support. Don’t rush into prep when you’ve only just finished thinking about and working on your thesis.
Wait before you prepare so that you can change gears from the kind of work that has completed your thesis to the particular work that will help you get ready for your viva.
Like any advice there are exceptions – say, if your schedule is so busy it won’t accommodate a break you might prefer to keep busy – but reflect on the intent behind the advice.
It will more likely than not help you to prepare better for your viva if you have a break after submission.