Here’s one caveat for you specifically: don’t train six days a week.
6 days a week of HIIT training, without enough time for your body to recover is a recipe for injury. Of course, this can vary from person to person and the intensity of the workout and the level of your experience and form - but these are things you didn't mention.
This would of course lead to injury. I don’t think anyone wants to squat heavy after sprinting.
Weightlifting is pretty safe when done right. Just make sure you have plenty of rest. At the same time, unrelated, but the posture benefits are way overrated. In fact, it’s gonna lead to bad posture over time unless you couple it with yoga or some sort of corrective stretching.
That's a fair call. Heavy weight training was usually on separate days, but I recall some crossfit WODs would include deadlifts alongside other fast-paced exercises, and now I question the safety of that. Those deadlifts were likely low weight, high-rep though.