A zero-cost alternative that has worked well for me so far is to use a front-end load balancer to distribute requests to multiple Phoenix instances (in k8s), and then just let those requests' background tasks run on the node that starts them.

The whole app is approximately a websocket-based chat app (with some other stuff), and the beauty of OTP + libcluster is that the websocket processes can communicate with each other, whether or not they're running on the same OTP node.