How uncomfortable is it to wing Peru in high season?

My partner and I are planning on visiting in July or August and we HATE planning detailed itineraries. Seeing as Peru seems quite logistically complicated, and we're going in high season, we're concerned about ending up in crappy situations of not finding tickets or accommodation because it's all been booked out.

The only thing we know for sure is that we want to do the Lares trek with 2 days of the Inca trail ending in Machu Picchu. We would book this in advance, and we've already confirmed they'd have availability. We unfortunately can't go to the Amazon, as I can't take the YF vax with my meds and it seems a bit risky to go unprotected - I'm still grieving this.

We're wondering how viable it would be to keep most of the trip relatively spontaneous. Does anyone have experience with this in recent years?