Your attic insulation is way subpar--walls and ceiling. You can increase the roof insulation on the interior or exterior (or both). For the walls, even high-density fiberglass batts are inadequate; fir out the wall. Change out single pane windows. And seal, seal, seal. While adding a supply duct, provide a cold air return. Before you start using the attic space as living space, check out the structure. It may well have been designed for dead storage only, using 2x6s that give you a floor that bounces like a trampoline as you walk across the space. Here is a link that might be useful: US Department of Energy Recommended R Values

If you don't put some backing on the framing you may have the insulation slumping and falling out the back, even if the facing is stapled in place. You could back the framing with foam insulation board that is glued or stapled to the framing. Since its on the cold side of the framing you don't want it to be a moisture barrier, so I think a product with Tyvek or other moisture-permeable barrier would be best. Bruce

Unless the panel is on the wall right next to the door, where you would need walking room to get to the "storage" anyway, it is hard to convince an inspector that a room that size can be used for both the panel and storage. Since both rooms are essentially bedrooms, it is also hard to convince an inspector that it won't end up a clothes closet at that size. I suppose if you ran home runs of Cat6 and coax into the room too, and called it a mechanical or utility room instead of a storage room, then the small size wouldn't matter. But, it is a judgement call or interpretation of the codes by the inspector. If the room was huge with plenty of 'other' walls to put storage shelves on, they usually don't care much about location. 110.26 Spaces About Electrical Equipment. Sufficient access and working space shall be provided and maintained about all electrical equipment to permit ready and safe operation and maintenance of such equipment. (30" wide × 36") (B) Clear Spaces. Working space required by this section shall not be used for storage. When normally enclosed live parts are exposed for inspection or servicing, the working space, if in a passageway or general open space, shall be suitably guarded. http://freenec.com/T13.html . 240.24 Location in or on Premises (D) Not in Vicinity of Easily Ignitible Material. Overcurrent devices shall not be located in the vicinity of easily ignitible material, such as in clothes closets. http://freenec.com/T66.html

One option would be to pull up those two sheets of plywood and use long tapered shims cut from a 2x4. Affix the shims to the floor joists and reinstall the plywood. (I would use some glue & a nail gun to affix the shims and then screw the plywood down with screws long enough to get a good bite into the joist. Pre-drilling through the shim to avoid splitting probably a good idea.). I would use a long level and figure out how much taper is needed before pulling up the ply. Number the shims if there is a variance in the taper. (To minimize having to teeter around on the joists.) Note that this approach may not be The Recommended way to do it. Just my initial thoughts on how to solve the problem to get the flooring 'close enough' for vinyl planks. I don't have experience with vinyl planks but have been shopping for flooring. There are some lines of vinyl plank flooring that are a stone & plastic composite that are supposed to be dimensionally stable over a wide range of temperatures. Have been following a long, long thread over on the flooring forum about the product. Here is a link to the product being discussed on the thread: Supercore Kryptonite There are other brands on the market that are a stone composite. Look for SPC (stone plastic composite.) Pricing is not a whole lot higher than your target price. Haven't fully researched the underlayment requirements so you'll need to ferret that out. Houzz thread where SPC flooring is discussed: [https://www.houzz.com/discussions/adura-max-vs-coretec-dsvw-vd~4249579?n=2496[(https://www.houzz.com/discussions/adura-max-vs-coretec-dsvw-vd~4249579?n=2485)