DIY low pitch metal roof with rigid insulation

Hello! After prowling this forum, reading manufacturer guides, and watching hours of Youtube videos, I am ready to think about DIY'ing a standing seam metal roof. The main part of the house is a gable roof with a 3:12 pitch. There is an addition that is about a 1.5:12 pitch with open-beamed ceilings that I would like to insulate from the outside. I'm in zone 3. It doesn't rain much here, but two years ago, it did drizzle daily in the winter.

3:12 pitch plan
-Has an attic, will be blowing in insulation to reach R-38
-Not currently vented as far as I can tell, considering ridge/soffit venting vs. whirlybirds. I've read that ridge/soffit venting may not be appropriate for such a low slope, but I don't know if that's true.
-3 Solatubes, would like to keep
-1 whirlybird over the garage, which is not insulated and I don't need it to be.
-Replace plywood
-Ice and water shield

1.5:12 pitch plan
-Open-beamed ceiling, haven't torn off roof to inspect, but it does not feel insulated
-Place plywood and ice and water shield
-2 layers of 1" rigid foam insulation

Following those steps in those part is where I am stuck: do I put down purlins and plywood? I was thinking just plywood on the 3:12 area since there is attic insulation and purlins then plywood on the 1.5:12 for some ventilation. Or does plywood go down first?

Much thanks in advance.