Yeah, it may be best to use both protocols. MQTT for the raw sensor data and Nostr events sent as periodic aggregate updates and alerts rather than as raw sensor data.
For my refrogeration project, for example, the MQTT sensor events are sent every 30 seconds. I use the quality of service setting 1 (at least once) otherwise sensor events are dropped, even with a direct ethernet connection.
Another program subscribes to the events and then sends reports and alerts via email and SMS over LTE-M and ethernet. Nostr could be an additional delivery mechanism.
Would Nostr also be useful for the raw sensor data?