You might want to consider removing the recessed lights, blowing in max insulation, and installing track lighting.
“Make a shoddy temporary covering for the recessed lights, which we don’t even use.”
If you don’t use them, get rid of them and install tracklighting. What kind of insulation do you have? Fiberglass? You know, there are companies that can blow foam into walls that already have fiberglass insulation, Vastly superior! Triple-glazed window, super low E. Get the foam guys to foam the garage ceiling/office floor.
Talk to a good “Comfort Control” company about the attic space and the venting. Perhaps another fan in that vent? More insulation. Re-routing the ductwork, etc..
My house is a slab, and the only vents are in the floor. Great for heating, as heat rises. Lousy for cooling, as cool air stays near the floor. What is your venting like? Same vents for hot and cold? When I remodeled one of the upstairs bedrooms, the venting from the furnace came up from the utility room and into the tiny attic over the dining room, and then was sent through floor venting to the baseboard mounted registers. When I redid the ceiling, I installed registers and venting for cool air. The entry to the diningroom attic is through a wall hatch, behind the dresser. I installed a “Y” valve on the vent from the furnace. In the Winter, turn the valve so the heat comes from the baseboard register. Summer,,flip the valve so the cool air comes from the ceiling registers. Works great!