Insulation for detached garage with trusses, Climate Zone 4C – GreenBuildingAdvisor

I’m looking for some suggestions/comments for insulating a detached garage with a modified parallel chord truss roof, as shown in the attached picture. 

The garage will be fully drywalled, and will have a minisplit system.  I’d like to get the right amount of insulation to help the HVAC, without wasting money (we’re in a mild climate 4C, Portland).

The builder has this insulation approach in the proposal:

-R-19 Unfaced 24×96 990730 PPS (B) Ceiling Insulation

-R-38 Unfaced 24×48 990425 PPS (T) Attic

– R-21 Unfaced 15.25×93 991185 PPS (T) 2×6 Exterior Walls

Options:

– R-23 INSULSAFE R MAX BIBS 2×6 651587 (T) 2×6 Exterior Walls

I’m thinking of accepting the R-23 option for the walls (+$895), which as I understand is blown in insulation, instead of batts.

But I’m not sure the ceiling/attic insulation portion makes sense.  I understand that we need to insulate the area right above the ceiling.  But since we’re not planning to use the “attic” space formed between the roof and the ceiling, do I need to insulate the “attic” (which I assume means the bottom of the roof)?  I think we can just leave that part vented, with soffit and ridge vents?

The picture shown in the attachment has insulation only along the bottom of the roof (none at the drywall ceiling), which I don’t think is a good approach(?).

Looking forward to some helpful advice and guidance – thank you!