When is it overkill to use a service mesh like Istio or Linkerd?

I'm starting a new project that'll live on Kubernetes, and my team's trying to figure out if we should bring in a service mesh right away. Part of me sees the good stuff it could do, like handling traffic, seeing what's going on, and setting up security rules. That'll be really helpful later, especially once we have a bunch of microservices. But then, it feels like a lot of extra work to manage all those parts, the control plane and data plane proxies, for a system that's pretty small right now. So, are there any simple rules or things about a project that make a service mesh an obvious good idea, or just too much trouble? And for smaller setups, are there other ways to get some of those benefits, like for metrics or routing, without jumping into a whole service mesh?