I am a beginning PhD student and the topic of my thesis is somewhere between real analysis, functional analysis and theoretical numerics. However, I am really curious about many (or rather most) fields of mathematics. If possible, I'd like to learn everything about mathematics, but of course I know that that's completely impossible.
How should I go about learning more about other fields of mathmatics that are not related to my thesis? I basically see two different ways how to approach this, although I am open to other ways I might have missed. Suppose I spend $n$ hours each day doing mathematics by myself. Is it better to spend those $n$ hours completely on working on my thesis, or to spend $n-k$ hours on my thesis and $k$ hours on learning some other topic, like algebra, topology, etc.? Suppose the latter option is preferable, what is an optimal value of $k$?
The reason why I am hesitating to spend time learning other topics is that I sometimes feel like wasting time, as I never know if I can ever apply this knowledge, or if I can even remember it when I could apply it. On the other hand, I don't think I will have more time later on in my career, so if I don't spend time now with other topics, I might never get a real chance to do so later again (except for retirement).
The same question for things that are related to mathematics but not mathematics themselves. For example, I am very interested in theoretical physics and AI.