Why will the wells dry up? If they’re 100’ or 200’ deep you’re saying they’ll dry up? Why?
Where do you think ground water comes from? It will simply run off. The ash becomes cement, non porous. Any rain will absorb about an inch, and just wash away.
And how are you going to pump? There will be no power. Anything sitting out on the lawn will be buried.
Really, think about six to ten feet of cement falling from the sky in an hour. Everywhere. And it’s boiling hot.
It’s not “lava” that will kill you. It’s ash plumes.
Again, this is all hypothetical. I don’t think it will happen. But the scale of a full blown caldera super volcano is one of those things that is literally incomprehensible.