Has there been significant progress in Go in terms of AI strength after AlphaZero (like there has been with chess), or has Go AI stagnated because of lack of interest and/or resources?
It's a bit hard to say for sure (since you can't pit them against each other, AlphaGo isn't available), but KataGo is very likely to be well stronger than AlphaGo ever was, unless maybe they contuinued it internally (unlikely).
If nothing else it's vastly more efficient to train and experiment with.