Customize Debezium pubsub message - debezium

Question

I am trying to use debezium server to stream "some" changes in a postgresql table. Namely this table being tracked has a json type column named "payload". I would like the message streamed to pubsub by debezium to contain only the contents of the payload column. Is that possible?
I ve explored the custom transformations provided by debezium but from what I could get it would only allow me to enrich the published message with extra fields, but not to publish only certain fields, which is what I want to do.
Edit:
The closest I got to what I wanted was to use the outbox transform but that published the following message:
{
"schema":{
...
},
"payload:{
"key":"value"
}
Whereas what I would like the message to be is:
{"key":"value"}
I ve tried adding an ExtractNewRecordState transform but still got the same results. My application.properties file looks like:
debezium.transforms=outbox,unwrap
debezium.transforms.outbox.type=io.debezium.transforms.outbox.EventRouter
debezium.transforms.outbox.table.field.event.key=grouping_key
debezium.transforms.outbox.table.field.event.payload.id=id
debezium.transforms.outbox.route.by.field=target
debezium.transforms.outbox.table.expand.json.payload=true
debezium.transforms.unwrap.type=io.debezium.transforms.ExtractNewRecordState
debezium.transforms.unwrap.add.fields=payload
Many thanks,
Daniel