How do I disable/configure the Transmission Module to send plain MQTT data payloads?
Transmission is fundamentally based on the Sparkplug specification. However, it is possible to publish as JSON instead of Sparkplug encoded protobuf messages. This is under Transmission → Settings → Servers → Settings → Advanced → Data Format Type
I should note that if you change this to publish as JSON, it will NOT be consumable by MQTT Engine or other Sparkplug Host Applications.
In addition to the above, it is possible to publish your own custom messages from MQTT Transmission. See this link for details: MQTT Publishing via MQTT Transmission - MQTT Modules for Ignition 8.x - Confluence
Thank you Wes for the quick response - I will look at the options you suggest.