Using MQTT Bridge and MySensors - getting started

Hello
I've set up MQTT bridge, from a legacy (MySensors) system which end-devices publish to, which now sends updates via MQTT to AWS-IoT.

I can now Test this by subscribing to topics - and can see the json arriving:
{
"topicFilter": "mygateway2-out/////+",
"qos": 0,
"messages": [
{
"format": "json",
"topic": "mygateway2-out/61/255/3/0/0",
"timestamp": 1620612201680,
"payload": 74
},
{
"format": "json",
"topic": "mygateway2-out/254/255/3/0/0",
"timestamp": 1620612198109,
"payload": 67
}
]
}

I want to be able to injest this into IoT-Core, so I can name, log and graph them. And also potentially "publish" to specific "topics" to control the devices.

I'm not sure where to start with this. Do I need to use SiteWise? Or can I create a lambda to transform these messages, and create "Things" in IoT-Core? Ideally, I'd like new "Things" to be created based on any new "topics" received. This would allow me to have MySensors added and Things created automatically.

I'm not using Greengrass or anything. Just Mosquitto as a broker with an AWS TLS secure Bridge function.

Can anyone steer me in the right direction?