This is why Dungeon Crawls suck at high levels.
Casters can, and will, dominate everything at that kind of level. As you seem to have worked out, Shapechange rules, and quite apart from that the kind of damage they can deal out in a very short space of time is astronomical. This only gets worse as levels advance and you run into the fun that is epic spellcasting.
The game is not balanced at these levels for the same kind of adventure that you would use at lower levels. It simply doesn't work the same way. You have to tailor specific challenges to specific group members, and you have to control things carefully so that it can work without others piling in and simply winning easily. You also need to get used to the fact that at these kind of levels D&D cannot be a combat-based game any more, if it ever was in you campaigns. Apart from anything else, it just gets ridiculous. How many dungeons full of advanced Half-Fiend Insectile Multi-headed Umbral Great Red Wyrms can there be in the world?
I think your mindset is wrong for such a game. If you want an idea of how it can be made to work, some Story Hours might help you. Specifically I think Sep's story hour (nicely compiled under 'Tales of Wyre' by Cheiromancer) is an exemplary model of how a near-epic and epic game should be run. It starts with the characters at about the kind of levle you are dealing with.
Of course, you might just decide it's not worth the effort. A lot of people don't think it is, and I can see why. Personally though I think it can be very rewarding.
Good luck!