when you do a metal roof you could do two trusses in the middle and use purlins to bridge them all this is how pole barns work not at all uncommon to have a 8-10 foot span between trusses on a pole barn.

This is how my house is done and how I redid my roof framing. The birdsmouth cut is actually done using the same speed square and is really simple once you understand what's going on. For a new shed, he just needs to look at the rafter tables to see how he needs to cut his ridges and birdmouths. Larry Haun has a great video on youtube that is about 40 minutes long but it really shows everything you need to build an entire house.

Speed squares are nice but I did all my rafter and stair layout with a framing square.

For a straight gable run lay out a pattern rafter make a duplicate, check fit, then use pattern to mark the rest.

I would go 12:12 it makes everything a 45 or right angle for cutting your birds mouth simplifies calculations plenty of slope to have a tall ceiling
I would also put a double header on the top of that wall to better distribute the weight if you don't have a rafter right over a stud.

I would also set up to hang over front and back and each side, more on the front to limit how much rain runs down the door , shooting that snow further from the building preserves your siding and foundation

are you going with steel siding also?

Just build yourself some trusses or have them built. 2x6 rafters and bottom chord. 24" centers. No plywood roof deck. All that does is add dead weight. Use 1x or 2x nailers every 16" and screw your tin to that. It will be plenty rigid once the metal is attached. Hurricane ties at the eave on every other truss.

Will be adequate for snow up to 30 psf live load and 10 psf dead load.

You really cant do a vaulted celin (uspside down V) without some sort of engineered parallel cord truss. I agree with white, you should see if someone can build you some trusses. It would take a while for that in my neck of the woods. Otherwise, throw up your gable rafters and ridge beam(another guy would be very helpful) then fill in your field rafters. Cut you collar ties/horizontal ceiling joist to your building’s spread. Might have to miter the ends for where the eave is. This will give you a consistent reference of your spread on the top plate. Some bracing would be good to before you do all this

I left my attic space open on my garage, put blueboard foam in between rafters for a little R and put sheets on top of collar ties for more storage. I did put 2x6 for collar tie because i like to store heavy items up there when out of season. (Traps, decoys, blinds, oars, etc)