I wasn't as clear on my original post as I should have been, my mistake.
I meant starting every 60-90 seconds, so the rest period is actually significantly less. I did this circuit on the weekend, with 7 exercises (because I was working out at home and did single leg so 2 slots to do legs). I started each new exercise every 1 minute 15 seconds, and 4 circuits took 35 minutes. The duration of work per 75 second slot was between 30 and 45 seconds. Even with a warmup and cool down which I have to take a while with due to past injuries, I was done in 55 minutes.
Problem is, as you correctly identified, it's over 7 minutes before I hit the same exercise although that is mitigated a little by the fact that most exercises are big movements which I guess work more than just their primary focus.
Your recommendation of combo exercises are a good option I think, I could superset with very minimal rest within the superset and still do it in about the same time.
I guess reframing the original question, what degree of hypertrophy can a GPP circuit achieve?