I wonder what your thoughts are on the above subject
1) To know how bad/abusive/ morally evil/sexist supervisors/academics can be (and academia in general).
2) To know other (ex-)students from the target institution and talk to them. This really helps!
3) To know how unimportant good schools/doing much work is compared to things like family, friends, health etc. Indeed, I've often wished to have done less work during the PHD and to have spend more time with friends and hobbies.
I hope this helps, maybe more later!!!!