You definitely know this world better than myself, but as first approximation I wouldn't try to provide storage, which we know how messy it can be.

Just keep a TCP connection open can go a very long way and it will get costumer to hook to redis or basically any other database.

But I don't believe >40 or so nodes has never been Erlang's strong point.

Edit: with the conclusion being of course that you wouldn't want to strongly connect the edge nodes, so instead it'd be something more traditional, where if people pay for a storage add-on, you folks are automating the schlep of spinning them up a long-lived storage cluster, and networking it so that only their edge nodes can access it. At which point you could be building on anything, maybe even something that already exists like Redis or etcd. Hmm, though for hello-world purposes I might still see what could be put together in beam-land, where everything is more at your fingertips ...