Re-roofing plan for double wide manufactured home in desert

Hello folks,

We live in the high desert North of Palm Springs and it's hot here... like 112 today and summer is just getting started! I'm a fan of metal roofs for the low maintenance and the high reflectivity possible.... but still confused as to what would be a cost effective retrofit design, with the goal of significantly increasing the R value of my existing roof structure. Moisture is just not a problem here, even dew is extremely rare.

The home's current roof is fiberglass shingles, over a simple 1/12 pitch rafter design (with maybe 10" total between inside ceiling and outside roof deck). I'm not completely sure what is in between... but because of the design (no attic), the only "easy" way to add more insulation would be add rigid foam on the outside of the existing roof deck.

I've read a lot of similar posts on this topic, but still unclear as to the recommended batten design and if air venting is worthwhile or not. Also I'm unclear as to how to handle trim issues on the eaves and lower roof edges. If anyone could point me to an online drawing/design for this sort of application... or provide recommendations I'd be very interested / grateful.

I'm only considering hidden fastener roof systems (I want to do this only once during the rest of my life...) and I'm planning on DIY, with some help from younger friends. It seems to me that removing the existing shingles and underlayment, followed by 1 1/2 or 2" foamboard and 1 x 3 vertical battens on top (fastened with screws through the foam) would be a good surface for attaching the metal roof clips and give a nice air gap for venting out the ridge line, but again looking for opinions and more details from the experts here!

THANKS!