First, realize that with a single zone system and multiple floors, it's difficult to get the temperature correct everywhere. Also, since you mentioned this was an attic space, you're likely against the roof and getting heat from every wall plus from the computers and people in the space. In short, you're fighting a losing battle.
I've got a similar challenge, though not to your extreme, and here's what I have done. First, learn how to adjust the baffles on the ducts. In the summer, you want as much as possible going to the high floors, and the reverse in the winter. Get someone to feel the air coming out while you're pushing the baffle all the way to one side or another. For me, it made little difference until I pushed it all the way.
Next, exclude any rooms you don't need to heat/cool, e.g. a guest room or storage space in the basement. Shut the door and close the vents to reduce the effort you're putting on the HVAC.
Finally, I swapped out my return grille with models that accept a filter (they are hinged and open after releasing two small clips). These return grilles are special orders from the big box stores, but they carry them. I still leave a thin blue "rock catcher" filter directly on my HVAC blower itself. And I put a filter in every return except the upstairs in the summer and downstairs in the winter. The idea being to suck out as much air as possible from location that needs it.
This means that switching between heat and cool is a process, flipping the baffles and swapping in and out filters. But a little effort is better than a lot of sweating.
One last suggestion, since it sounds like you're running a small server farm in your attic. Either move that down to the basement, or consider replacing the window unit with a ductless mini-split system. You get to see out the window, and these systems have configurable thermostats.
All the things you've done should have helped some, except the duct boosters. They can't add much flow to the system unless the return ducts were increased proportionally (or similarly boosted). You get increased flow by increased pressure or larger ducting. Keeping upstairs doors closed will help by slowing the 'fall' of cooler air downstairs. Try leaving fan "ON" for a 24 hr period to see if your house will equalize better.
You can force the system to concentrate on the upstairs by reducing both the supply and return vents in the main floor and basement. You may have to do some makeshift blocking (reduction
) of the returns with plastic sheeting and 'no residue' duct tape. If successful, I believe magnetic covers are available.
AFA blocking above the upstairs ducts, by all means. Save yourself some expense (and trouble, that much foam is hard to scaffold in a vertical shaft) and make 2x4 blocking (essentially fire blocking) to go above the vents. If you have typical framing 16 inches on center, a 14 1/2 inch piece will span stud-to-stud. Use a pair of pocket screws on each end to secure the blocking in place. Use fire caulk/foam for any gaps.
Best Answer
Years ago, every room had a cold air return. Today, most HVAC systems have a central return air grill. In order to get air delivery into that room, the bottom of the doors need to be trimmed. I am from the "old school" and still believe in a return grill in every room except the bath room and the kitchen. Why the central return grill, installed cost, of course. Now we see questions as to "how can I reduce the noise coming into a bedroom"? Easy, insulate the walls, add a return in that room, and replace the door which has had the bottom cut off to enhance air flow. My 2 cents.