The normal way to do this is to install blocking between the rafters (for the ceiling drywall backer) and between the studs (for the wall backer). You don't need to toenail all of them... you can screw about half of them.
Easier from a building perspective, run full length 2x4s (or equivalent) top, middle and bottom of the wall (assuming it's under 4' -- if it's taller, then 2 middles). Run 2x4s on the underside of the rafters on 16" centers. You've probably figured out by now that this is a ton of lumber with the added non-benefit of less habitable space.
Neither of these options require bevel-ing. A tiny gap at the edge of the drywall doesn't matter.
If the underlying problem is the cutting, rent a miter saw for half a day and cut your blocking in one swoop.
We worry about verticals loading AND horizontal loading. Resting the joists on the double top plate works great for vertical loading and using hangers to keep them aligned until the loft floor sheathing is installed is a great idea, because you can’t end nail them through the rim joist. (A cheaper clip, like Simpson’s A34 or A35 on one side of each joist would work too.)
However, I’d worry about 1) spacing of the joists, 2) securing that overhead load to the vertical supports (stud wall), 3) keeping the loft from rotating or moving away from the main building.
1) You said, “above you office”, so it’s not residential storage. The Code requires a minimum of 125 lbs. per square foot for light loads and 250 lbs. per square foot for heavy loads. (See ICC Table 1607.1.) That means those 2x6’s need to be 12” oc to span you 8’ loft.
2) You don’t have to design against wind loading, because you’re inside another building. However, depending where you’re located, seismic loading could be severe and you can’t rely on toe-nailing to secure the loft framing to the studs.
I’d use either a clip from the joist to the top plate or studs at about 24” oc or I’d use plywood sheathing to hold the joists to the top plate (and keep the loft framing rigid...but more about that in item 3) below. )
When you have a lot of weight up in the air, an earthquake can get (and keep) it moving. Gypsum board (wallboard) can resist this movement, but it’s fairly weak. OSB board or plywood is much better. If you told me you were going to store your Christmas decorations up there I’d thing gypsum board would work. But if you are putting books, dead files, or that old transmission you were going to fix someday, up there, I’d worry....then you’re well into the 250 lbs. per square foot.
3) The whole loft needs to be rigid enough to keep everything in the loft off your head while you’re preparing your will. I like 1/2” plywood nailed at 6” oc to all WALLS and all supports, including the loft rim joists. That way you solve item 2) above, and item 3).
Fastening the office and loft to the main building is a good idea, but I’d need more info on type of building, height, material, etc.
Best Answer
In my opinion this should be done 3 times per span at a minimum. So really it depends what you are building with. If you use your stock 2x4s that are 8 feet then 3. But If you get longer 2x4s I think you can also go with 3, given there are no breaks.
(This is also assuming that you can fasten the bottom every couple feet)