If you're not a dualist, thoughts necessarily have to be accompanied by some change of state of the material substrate.

Don't underestimate the complexity of a rock's internal state. There is a unfathomable number of atoms in complex crystal lattices in there. Defects in that lattice alone can encode a ton of information. Then you have a bunch of quantum state for each elementary particle. All of that reacts to external stimuli like temperature, pressure, or the occasional cosmic ray.

Thoughts are a process, so if they were supported by a crystal lattice, that lattice would have to go through changes.

As to your other point, the ergodicity of the system at the very least places a limit on the timescales over which consciousness may persist. I'm fairly confident in my belief that a rock cannot be conscious in any appreciable way.

You assume that dualism and materialism are the only options. You ignore other possibilities, such as idealism and neutral monism.

You're right. The dividing line is whether or not you think the physical and the mental necessarily coincide, which is in principle an orthogonal issue.