Do you know when implementation of JSON column will be available?

You can use this for now, if you want:

class JsonStore<T : Comparable<T>, S>(
   private val json: Json,
   private val serializer: KSerializer<S>,
   private val column: Column<String>,
) {

   operator fun getValue(thisRef: Entity<T>, property: KProperty<*>): S =
      json.decodeFromString(serializer, with(thisRef) { column.lookup() })

   operator fun setValue(thisRef: Entity<T>, property: KProperty<*>, value: S) =
      with(thisRef) { column.setValue(this, property, json.encodeToString(serializer, value)) }
}

inline fun <T : Comparable<T>, reified S> jsonStore(column: Column<String>, json: Json = Json.Default) = JsonStore<T, S>(
   json,
   serializer(),
   column,
)

Ok, I will test it