Yes, I think NATS is reasonable — I don't have operational experience with it but based on my earlier reading it seems that it can be run on a smaller budget. Is this "regular" NATS or the Jetstream variant?
Perf! From testing on some of my staging workloads, the footprint isn't too high and I can get 5-6k messages/s. Esp. since there is only one worker instance involved (for strict ordering).