Spells like Solid Fog don't allow a save, and don't permit SR to interfere... with Shadow Conjuration, they would. It makes a "No Save, you just suck" spell into a "Will save or suck, but only in a limited area", which makes it worse than Slow. So you blow a 7th level spell (Greater Shadow Conjuration) to produce a 4th level effect (solid fog) that is nerfed to the point where it would be better to cast a 3rd level spell (Slow). How is that an effective use of magic?
Besides, 7th level has Mordikain's Magnificent
MansionLove Shack. It's like Rope Trick's bigger brother without the hassle of having to shinny up a rope, and with efficient staff that comes entailed with the property! While not directly combat effective, it gives you a 'reset button' on your spells. When you run low, pop your MMM and take a nap, then come back out refreshed and renewed for the next day of murder and mayhem.
Greater Teleport is also a 7th level spell, and one you should 'trade up' for.
Forcecage is a trap. It costs 1500 gp per casting. Use Greater Shadow Evocation instead. While it grants a Will save (if he makes the save, the forcecage vanishes) it doesn't cost components, and like you said... with the sky-high saves on your illusion spells, it's not going to be easy to make.
Prismatic Spray is another good one, since you are a Sorcerer and not a Wizard who banned Evocation. Roll higher than a 3 and it's a Save or Loose effect. Roll an 8 and it could be TWO Save or Loose effects. In a Cone-shaped burst. Cone of LOSE!
Finger of Death and Reverse Gravity are also nice 7th level spells to have.