I'll try to explain what the house has without making a huge wall of text...
The house is a split level with a protrusion off the front that has half the basement and the master bedroom. Half of the main attic is a finished room (left side of the pic).
Walls are mostly uninsulated except for the ~20ft in the kitchen (had the drywall down, installed R15). Brick facade, a layer of fibrous sheathing, open stud wall otherwise.
Ceilings facing the attic have presumably 1954 vintage fiberglass batts a few inches thick (R13, 15, who knows) plus unknown vintage loose cellulose which seems to be 8-9 inches thick except a small area where the whole house fan has moved it. The sloped parts of the attic room ceiling just have the fiberglass with an air gap between it and the roof sheathing. The attic over the master bedroom is very low, under 3 ft from rafters to peak. There is a similar height tunnel above the attic room.
The bedroom attic has 2 turtle vents in the roof and the main attic has a continuous ridge vent and 2 gable vents. Both attic areas are open to each other. The soffits are fairly narrow and were unvented but I recently cut them out so air inlet is as good as it's going to get. From what I can tell the cellulose is now now longer blocking any airflow and almost every rafter space has a clear path between the soffit and attic area.
The problem: on really hot days like this recent east coast heat wave, the attic insulation heat soaks right through by 3pm or so especially over the master bedroom. Of course the attic room also gets hot very easily. From what I can tell, the fiberglass plus all that cellulose should have a reasonably good R value.
Should I be considering redoing/adding to/whatever the attic insulation or not? Otherwise, I know I should probably seal off the gable vents to let the soffit/ridge system do its thing. Also the roof over the bedroom is a hip roof so I could block the turtle vents and probably get 6ft or so of ridge vent installed instead. If I get really motivated I may take drywall down in the attic room to put better insulation in the sloped ceiling part.
Any help would be greatly appreciated!