I agree with @metaltyphoon on this. Even for small teams, a managed version of Kubernetes takes away most of the pain. I've used both ECS+Fargate and Kubernetes, but these days, I prefer Kubernetes mainly because the ecosystem is way bigger, both vendor and open source. Most of the problems we run into are always one search or open source project away.
Are you assuming the workloads have to use K8s APIs? Where is this coming from? If that’s not the case can you actually explain with a concrete example?
Man, you don't need to use service mesh just because you use k8s. Istio is a very advanced component that 99% of users don't need.
So if you are going to compare with a managed solution, compare with something equivalent. Take a bare managed cluster and add a single Deployment to it, it will be no more complex than ECS, while giving you much better developer ergonomics.