When should you actually start using a service mesh

So, we've been using microservices for a bit, and honestly, it's getting messy. Dealing with how services talk to each other, mTLS, retries, and just keeping an eye on things is a huge pain for each team. I'm looking into service meshes now, like Istio or Linkerd. Have you guys used these? When did you feel like the extra complexity was worth it, instead of just building all that stuff into each service?