Well, we know that the simple fact of observing an event changes it (see the Double Slit experiment), so consciousness has to have some kind of link to reality itself, no?

I think you might be misunderstanding what “observation” means in that context.

Wikipedia: Observer Effect

In physics, the observer effect is the disturbance of an observed system by the act of observation. This is often the result of utilizing instruments that, by necessity, alter the state of what they measure in some manner.

Your opening statement is incorrect. Observation in the quantum mechanics sense does not have anything to do with consciousness. Observation is really just a form of interaction.

We can't rule out something as impossible just because we haven't observed it yet, as it would directly contradict the scientific method

Figuring out what's possible versus impossible isn't really part of the scientific method. The scientific method is about collecting and interpreting evidence. Where is the evidence that particles are conscious?

Until there is a testable hypothesis, panpsychism doesn't have anything to do with science.

Others in this thread have already explained that consciousness doesn't play any role in the double slit experiment. I definitely understand your confusion there. I believed the same thing at one point. It doesn't help that some people purposely spread that false interpretation of the experiment because it's more interesting than reality.