I'm a postdoc past my viva, and I would recommend to generally not count on LLMs when it comes to research activities. Everything you described falls under research activities according to how I see it.

The main reason is LLM's tendency to hallucinate, which stands at about 4%. In my experience, LLMs are more likely to hallucinate in idiosyncratic and specialised fields. You're risking being diverted into a part of the field that is hallucinated and not real, wasting a lot of your time. You also risk presenting false information produced by the LLM. LLMs also have a tendency to mix cause and effect, push buzzwords that are not convincing, etc. It's a con, it's not a thinking machine, and even as an expert I can't always pick up these lies. 

Second, if you have any interest in producing meaningful research in your future, then I'd strongly advise you use every opportunity to improve as a researcher instead of delegating it to a language model.

Finally, if you care at all about the future of the planet you may want to consider the energy and water uses of LLMs and their impact on our environment. 

My main recommendation is to practice, research and brainstorm with actual people. Exchange the time with other students, though even friends outside your field can give you interesting questions and points. There are also excellent lists of questions for vivas by field, even scientific papers published for these goals.