I would guess most people don’t really “hike”the Canyon. The rim trail is basically walking a paved trail. Once outside the Village (hotel/restaurant area) it is more hilly but there are buses that run up and down the trail. I have a bum knee and hiked into the canyon for all of 10 minutes, then back up. We did walk the rim trail back and forth past the hotels and gift shops then took buses the rest of the time. The weather at the rim was pleasant, 75-85 and breezy most of the time, cool at night. I wouldn’t go out hiking far in mid afternoon (took that time to relax and have ice cream) but otherwise it was good. Phoenix however was 105+ and I wouldn’t enjoy that at all.
As for peak season, I’m not sure April is peak in the Canyon (some seasonal stuff doesn’t open til May there) but they book 11 months out, so you can check to see what’s available now if you want to stay at the rim (grandcanyonlodges.com) We stayed at the Hampton Inn Flagstaff East which has been open maybe 6-8 months and is very nice. Also stayed in Williams AZ, which is on Route 66, one night.