If you’ve nothing left when you sit down to get ready for the viva, then you’re not ready to get ready.
Take a break.
Submit, then stop.
Pause, then prep.
Relax and restore, then ready yourself. You can’t get ready for the viva if you have nothing more to give.
A few weeks or even a few days can be enough to recover from that final push to get your thesis finished and submitted. Viva preparation does not require fantastic efforts either, so if in day-to-day life, after work and other commitments you only have a little, then only give a little. You don’t need to ruin yourself to get ready.
Rest, restore and refuel first.