I have pretty much retired (i.e. suck at interviews and can't deal with the nonsense in industry anymore and am not one for management) and am thinking of going to get a PhD just because these days, I like doing research for the hell of it. Someday I will finish writing my first paper.... If I ever get a full time job again, it will have to be some sort of researchy sort of thing, so a PhD will be useful to get past HR etc.

Your PhD program is what you make of it. If you are not interested in going into full time academia, the uni will still take your money if you look like a good candidate and can get through the program. I suppose you have to worry about competing with other students if you both are trying to get the one open position with a professor... So make sure you can pay your own way and then you won't be dependent on being a wage slave for the university and all that industry-lite crap.

But really, ask yourself over and over "why do I want a PhD?" until you're sure of the answer - it is 3+ years of your life doing only that and it could be brutal due to the workload etc.

Engineering/CS programs are probably more laid back or less 'political' (i.e. those scary stories) than humanities or other STEM degrees. I have noticed some interpersonal drama in some sense just hanging around the uni these last few years, e.g. students and dealing with them, a general sense of the academic environment. But I make sure to stay out of it.

Only a minority of CS PhDs pursue a tenure-track position. I have found academia to be more laid back than big tech companies, but you are still expected to produce (e.g., a paper or two a year). The top programs probably have a more competitive culture than state schools. If you don’t want to produce research, then I don’t see the point in enrolling.

I [luckily] haven’t experienced the scary stuff that people talk about, but a major barrier for students is that PhDs are largely unstructured and require you to take the initiative. Not everyone does well in that environment.

How about teaching undergraduates? I seem to recall from my undergrad days that a lot of my TAs were pursuing PhDs. Is that something that PhD students are expected to do?

Some work as TAs. It is done part-time for funding (tuition waiver, monthly stipend, and insurance). It is not usually a requirement in US schools, although I recommend my students do it for at least a semester to get the experience. Otherwise they are funded as RAs.

A PhD is about doing research.

How easy is it for someone to go back and just get a PhD. After recently being rejected from a handful of schools (albeit undergrad, not graduate) I’m starting to feel like academia only wants people who played into their game from the start.

There are plenty of programs that think about things like quality of life, etc. Culture and competitiveness is a function of the school itself and the PI specifically.