That was always my assumption about why it happened, but it turns out that’s not the case at all: https://youtu.be/CUjt36SD3h8

In aggregate, yes, but any individual wave of light is still traveling at c. You get the appearance of a slower wave because secondary waves are generated that cancel the original one in such a way that it makes a combined wave that appears to be slower.

I’ve been trying to wrap my head around this part and what it means for cause and effect for a while. I think Feynman said something like a photon is only ever emitted when the source and destination agree to exchange one. Which makes sense if the exchange is instantaneous to the photon. But how can billions of years pass for us in the mean time?

To be pedantic, photons never accelerate. They only ever travel at one speed in one direction