it's commonplace to hold the opinion that an individual particle is slightly conscious

This is a thing... but it is certainly not commonplace.

In philosophy of mind it's not something that would raise an eyebrow

Are you referring to pansychism? I don't get the impression that it's very popular these days.

> There are few explicit defenders of panpsychism at the present time. The most prominent are David Griffin, Gregg Rosenberg, David Skrbina and Timothy Sprigge.

https://plato.stanford.edu/ARCHIVES/WIN2009/entries/panpsych...

But that’s my point! When was it conscious? A million cells? A billion?

My hunch is that consciousness will come to be described in terms of information processing systems, rather than the biological substrate on which they run. Systems with certain properties - possibly including a sufficiently complex model of the environment, model of the self, and a continuously updating internal narrative - may be said to possess a level of consciousness. I'm not a biologist and I appreciate that the above is somewhat vague, but I wouldn't say that sperm or egg cells have the above.

It's the problem of heaps[0] and I think you'll find Michael Levin's fascinating perspective[1] on this topic to be very interesting

[0] https://en.m.wikipedia.org/wiki/Sorites_paradox

[1] https://youtu.be/e5GDnD_WA2Q?si=QQfWr3HsHQarhk-H