Yes, if things expanding at 0.1c are common, then we should see galaxies containing them, but would we notice them? Would the galaxy look unnatural from this distance?

This would depend on what exactly they are doing to those galaxies. If they are doing stellar engineering (e.g. making Dyson spheres, Matrioshka brains, stellar lifting) then we'd probably notice if it were any nearby galaxy. But conceivably something might try to deliberately hide its activity.

Not directly relevant, but I'm not sure how you're using filtration. I use it in a Fermi paradox sense: a filter is something that explains the failure to expand. An expanding filter is thus nonsense. I suppose you could use it in a doomsday argument sense - "Where does my reference class end?" - but I don't think that is usual.

Yes, I think I'm using it in some form closer to the second. In the context of the first one, in regards solely to the Fermi problem then AGI is simply not a filter at all which if anything makes the original point stronger.