“Fortunately not all programs are so solitary, but I’m guessing that is the norm.”
Mine was incredibly solitary, but the UK system is very different, just three years of pure research. So solitary that my advisor went away on leave for my final year and just left me to get on with it. I like being solitary - I work for myself nowadays and my favorite activity is hiking.
Fortunately a math PhD can be done with very little actual “work”, just some inspiration, which comes best when you are sitting in the bath or doing something else. A friend (who is now a professor at Oxford) told me a good math PhD needed 2 hours of inspiration over 3 years, so I used to count minutes. I never got to 2 hours, but I was done in six years total (undergrad+PhD) and could then go and get a real job. I never had ambitions to be an academic so the PhD was essentially just a pleasant excuse to spend three more years enjoying myself in college. Not depressing at all. The US system (and the whole academic career track) seems pretty traumatic in comparison.