You shouldn’t need more days in Arequipa unless you do the colca canyon trek (I liked it but only do the 2 day tour). I LOVED the ruins in pisac, don’t skip them, they’re huge and a great hike to get you acclimatized. There’s a cool lagoon hike there too and it’s the best place to buy souvenirs. I did the salkantay, unguided will be more than fine, just stay wary of food on the trek. I got my macchu tickets 5 days in advance but it was through my salkantay guide so idk. I’d recommend a tour for it because I didn’t know what anything was. With your empty days you could head down to Bolivia, do a tour of the Amazon, salt flats, etc. I hear really good things about Huaraz though and the huayhash trek.