Suppose there are boards B1 and B2. B1 has child boards B11 and B12. Each board is represented as a folder with the same name. The main foder, F, could be structured as follows (every instance of content.txt represents a file; the rest are folders).

F
├───B1
│   ├───content.txt
│   ├───B11
│   │   └───content.txt (*)
│   └───B12
│       └───content.txt
└───B2
    └───content.txt

Now here's what a content.txt file could look like. Suppose we're looking at (*).

{
    "name": "B11",
    "topics": [
        {
            "topicId": "1111",
            "subject": "Help me out plz",
            "op": {"username": "ltcltcltc", "activity": 26, "merit": 60},
            "time": <timestamp of the original post>,
            "messages": [
                {
                    "msgId": "6666",
                    "author": {"userId": "3596085", "username": "ltcltcltc", "activity": 26, "merit": 60},
                    "time": <timestamp of this message (in this case the original post)>,
                    "merited": "2",
                    "message": "Hey does anyone know how to speed up ecdsa signature bruteforcing?"
                },
                {
                    "msgId": "6699",
                    "author": {"userId": "3597570", "username": "aleph1", "activity": 1, "merit": 23},
                    "time": <timestamp of this message>,
                    "merited": 0,
                    "message": "Stop wasting your time."
                }
            ]
        },
        {
            "topicId": "2222",
            "subject": "Test. Do not answer.",
            "op": {"userId": "3597570", "username": "aleph1", "activity": 1, "merit": 23},
            "time": <timestamp of the original post>,
            "messages": [
                {
                    "msgId": "8008",
                    "author": {"userId": "3597570", "username": "aleph1", "activity": 1, "merit": 23},
                    "time": <timestamp of this message (in this case the original post)>,
                    "merited": "3",
                    "message": "Testy test."
                }
            ]
        }
    ]
}


I didn't give any example of timestamp because I don't know what your time format is, but I think I'd prefer Unix time. Also note the redundancy: the topic's timestamp is the same as the timestamp on the first message of said topic. The topics inside each board are ordered chronologically (older first) and the messages inside each topic too.

What do you think about this format?