My point is more that expansion of the strong AI would not occur at the speed of light

How do you know that? Or why do you think it's a reasonable assumption?

so there should be very distant but observable galactic-level civilizations of AIs changing the very nature of the regions they reside in, in ways that would be spectrally observable.

How would we tell if a phenomenon is natural or artificial?

in those multiverses where a local AI respects some sort of prime directive, we may be left alone our immediate stellar neighborhood should nevertheless contain signs of extraterrestrial resource usage.

It would not be a good implementation of the prime directive if the signs of superior intelligences were obvious.