You can’t be overprepared for your viva, but you can be overinvested. You can do too much by doing more than is needed.
Through uncertainty or worry you could easily spend more hours, do more tasks or obsess more than is required.
Keep it simple. Plan ahead. Don’t fill your days. Figure out the core tasks required and do them when the time comes.
Not too much, just enough – just like your thesis, your research and you.