Do I need Ambassador if I use Istio for Mapping?
Ambassador is essentially a Kubernetes ingress controller/API gateway that uses Envoy.
And Istio is a service mesh that happens to have its own ingress mechanism, and Istio also uses Envoy as sidecar proxies. So in a way yes, you can replace Ambassador with Istio, but you may not be able to get the API gateway type of features with Istio. This blog explains the differences and attempts to clarify the confusion.
I'm not really sure that Istio is going to help you with the logs since it looks like the issue that you are seeing with log spamming is related to the API gateway component. It really depends on how you architect your application/services.