I think you can use Discourse API for this.
I did the same for that about 20K topic, can be done in hour.
I was using JAVA Test to make API call, the most important code as below:
CloseableHttpClient client = HttpClients.createDefault();
HttpPost httpPost = new HttpPost("https://www.ossez.com/posts.json");
httpPost.setHeader(HttpHeaders.CONTENT_TYPE, "application/json; charset=UTF-8");
httpPost.setHeader("Api-Key", "XXXXXX");
httpPost.setHeader("Api-Username", "XXXXXX");
TopicRequest topicRequest = new TopicRequest();
topicRequest.setTitle(bbsOssezForumPost.getSubject());
topicRequest.setRaw(postCtx);
topicRequest.setCreated_at(new DateTime(bbsOssezForumPost.getDateline() * 1000L).toString() );
topicRequest.setCategory(30);
StringEntity postingString = new StringEntity(new Gson().toJson(topicRequest), StandardCharsets.UTF_8);
httpPost.setEntity(postingString);