Hello, I was hoping to get some feedback before jumping in to this.
I would like to insulate my 2-car garage, and I think my requirements slightly more unique than a standard application.

It is a 24’x24′ garage, located in zone 5A.
Currently I have started insulating the walls with R15 rockwool.
For the roof, I have 2×4 trusses, 24″ OC. Soffit vents, and gable vents at either end.

What I am thinking for the roof is to insulate with 1.5 or 2″ foam board between the top chords of the trusses. I would do this up the roof line, until just under the gable vents, where I would go across, creating a “mini attic”, just enough for the soffits to vent up there, then out the gable vents.
Then I would add a layer of rockwool under the foam. Since the foam board would take up the remaining depth of the 2×4 truss, I would need some structure to retain the rockwool batts. I have a few ideas for this. Also, I would need to work around the existing joints in the trusses.

I believe the typical approach would probably be to drywall the bottom of the trusses, then insulate above that, a more proper “attic”. A few reasons I would like to go with the described approach instead-

The last two trusses on one side are scissor trusses, to allow clearance for a door the is a couple feet up. This would require added complexity for drywall and insulation to make a small wall between the mismatch of the flat truss and the scissor truss.
I also prefer the openness for a few reasons minor but practical reasons.
Also, I was planning on leaving the rockwool in the roof exposed, other than whatever structure is holding it up there. A huge benefit of this for me would be acoustic damping. Thermal insulation is top priority, but any sound insulation I can get would be a big plus.

Any feedback or critiques are greatly appreciated. I can provide pictures or drawings as well, if at all helpful. Thanks in advance for any help.