9 & 12 sets is close enough, still moderate sets.
Keep in mind that this is just my experience, after years of doing volume. I go to positive failure all of the time, but not in a gruelling way, just going to the very last full rep. Some claim going to failure isn't necessary, which doesn't make sense to me.
What you're doing sounds good, given that you're already doing moderate sets, not volume-going to failure almost all or all of the time, then on the last set of each exercise, to negative failure. The only thing is the negative failure part-if you're going to positive failure on the other sets, going as far as you are into negative failure on the last one sounds gruelling. Bottom line's progress, if you can handle it and are improving, it's good. If not, I'd pull back from negative failure some of the time, to maintain energy towards doing more reps or more weight.
One other thing to consider would be to remove one exercise from each muscle-do 6 sets for arms, 9 for lats/chest. Try for a few workouts, but using the exact approach you're using, and see if less sets allows you to focus more on increasing weight or reps.
Another thing to try is compound supersets, hitting the same muscle with two consecutive sets, with only 5-8 seconds between sets. In this case, very little if any negative failure reps!
Have you tried alternates to negatives-partials or rest-pause reps..I find them less gruelling than negatives or negative failure.