Does any researcher/scholar/professor here ever feel stuck with the field they have chosen to pursue? Do you ever feel it's too late to switch or regret not choosing something else while studying?
No, not really, but I do regret that one life time is not enough to learn everything I find interesting, even when just considering mathematics, let alone everything else. So, I hope for immortality (without dementia) or reincarnation (into a reasonably intelligent life form).
I couldn't find a job I enjoy with what I graduated, so I self studied and made a successful career change with some luck. I do wish I chose something different in University but there was no way to know.