The four-byte session expiry interval in the CONNECT packet is currently not optional. In MQTT 5, it is optional:

If the Session Expiry Interval is absent the value 0 is used. If it is set to 0, or is absent, the Session ends when the Network Connection is closed.

I think it should be optional in MQTT-SN also. This can save 4 bytes in the CONNECT packet. We have one connect flag left so we can use that to indicate presence. I would shift up the Default Awake Messages 4 bits so that this flag sits with the Auth and Will presence flags.