Vaulted Ceiling w/shingles, metal roof thermal barrier

I have an awkward little house in Phoenix, AZ. Imagine an A frame then cut it in half and add a little room above it. The 80's were a strange time. The majority of the roof is one big south facing plane. Brilliant, I know, lol. To make matters worse, the whole house has vaulted ceilings, very little insulation and no attic on the south side. It has a single layer of shingles from 1983. My dad and I would like to get supplies for a metal roof and DIY it. He is a contractor and Journeyman. What would be the best way to insulated it? We are thinking white tin on purlins and battens with spray in foam. Should we remove the shingles or put it all on top. I saw hard foam metal panels online, but it looks like they are only for commercial buildings. I work for a solar company now and we are planning on putting solar panels on top. Having a large south face works to my advantage for solar, but it is at 45 degrees. Weight isn't really a concern with solar, the panels are so light. But the lag bolts for the racking that holds the panels is a concern. We were thinking of an air pocket, but with bees and rats in Phoenix, it would be hard, even with netting to keep them out. The highest peak is where the air handler is, which is terrible, lol. It has no insulation or drywall in there. I am planning on adding radiant foil to the inside where the air handler is.