I'm also restarting this AP after a break. We are scraping mythic as well.
I've heard of the 25th level cap for non-mythic play, but I'm still wondering if 20th level is enough. When my players reached the Ivory Sanctum, their level was per the AP and they were getting nailed by the monsters. I never experienced the issues everyone else experienced. Our battles were very hard.
We still did experience the "rocket tag" more than at 6th level, but that's usual for the game. The issues we had were more about the rules, the action interrupts, etc.
I've been reviewing the battles in book 6 vs 20th level players, I think it's a possibility.
Do you not think they players could survive 20th level and this campaign?