There’s always more.
More research. More prep. More you could say in the viva.
At some point you have to say enough.
You have to figure out and decide when you have enough research and you’ve done enough in your thesis. You can plan in advance, then do the work to know you have completed all the prep you need to be ready. And while you won’t get to tell your examiners that they’ve heard enough, you will be able to communicate what you need!
Still, there is always more. You have to accept that and feel confident that what you have is enough.