Looks like a lot of fun to me. Personally, when I DM, hit points are more of a guideline than written in stone. I'll lower or raise them as necessary to make it fun. What is far more interesting to me is that the Donjon abilities aren't just "take out the threatening PC" abilities - they are plot hooks! It wouldn't be the first time I've had PCs trying to intentionally get swallowed in order to get something they needed. Given how long these things probably live, there's got to be plenty of interesting things (or beings!) in their stomach planes.
With a low intelligence, I can also see PCs trying to lead a major attack on a githyanki fortress and having the githyanki herd one of these to rip through the incoming forces.
Actually, considering that when it dies, everything from inside the Donjon appears around it, if you could have some control over where it goes, it makes a nasty Trojan Horse - either used by the PCs or against them. (The barbarian re-appears covered in numerous claw and bite marks, and says "I was just teleported into some cave -" The wizard interrupts, "It's the beast's donjon - basically a demiplane in place of its stomach." The barbarian continues, annoyed, "Whatever it was, I couldn't see what was in there, but there were a lot of them, and they were NOT nice!")
Toe-to-toe high level battles of just reducing hit points as fast as possible always seem boring to me. I really like the astral dreadnought because it's so much more than a bag of hit points, and can be used in some really creative ways. For really high CR battles, it's best to have some other goal than just getting it's hit points to zero, and this creature has some nice hooks for that.