It was brand new at the time come to think of it, it wasn’t released until 2008 so this more likely happened in 2009. The timing and the dramatic difference from stock to jailbroken is just too striking to have been a coincidence, although you might be alleviating some 15-16 year old guilt, that perhaps it triggered something. Still very worrying that a new and very expensive phone was triggered in to dysfunction from the process but maybe it was unlucky defective model. I definitely think that while it was jailbroken the problems were as a result of the OS but maybe the Cydia apps or something else were particularly draining and then that fast draining cycle triggered something else physically.