The NATS protocol [0] looks pretty simple, but is text based unlike MQTT which uses a size optimized binary protocol.

[0]: https://docs.nats.io/nats-protocol/nats-protocol

NATS actually also has support for MQTT[0]. If you already have a deployment for queues, microservices, etc, using the NATS mqtt endpoint seems like a very good option.

[0] https://docs.nats.io/nats-server/configuration/mqtt