do you have control on how the mqtt payload is constructed? As it would be best to send the string in json format, then you would not need to do any thing to convert it.
If you can not change the mqtt payload, this should work
[{"id":"8b0f0b705735122d","type":"inject","z":"bf9e1e33.030598","name":"","props":[{"p":"payload"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"\"TT001: 56/TT104: 56/TT105: 56/TT106: 56/TT201: 56/TT202: 56/TT301: 56/TT302: 56/TT303: 56/TT304: 66/@MYSITE","payloadType":"str","x":150,"y":3280,"wires":[["b238257.6f33858"]]},{"id":"b238257.6f33858","type":"function","z":"bf9e1e33.030598","name":"","func":"msg.payload = msg.payload.split(\"/\").slice(0,-1);\nmsg.payload = msg.payload.reduce((acc,str) =>{\n let arr = str.split(\": \");\n return Object.assign(\n acc,\n {[arr[0]]:arr[1]})\n},{})\nreturn msg;","outputs":1,"noerr":0,"initialize":"","finalize":"","libs":[],"x":360,"y":3260,"wires":[["9c792ae0.1bc0b"]]},{"id":"9c792ae0.1bc0b","type":"debug","z":"bf9e1e33.030598","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":640,"y":3300,"wires":[]}]