Hi 
If you want to create new topics, I’d use https://docs.discourse.org/#tag/Topics/operation/createTopicPostPM, and set a short time limit between each request to prevent reaching the rate limit.
I think you could also use a rails script and use https://github.com/discourse/discourse/blob/main/lib/topic_creator.rb. You can have a look at the importer scripts to see how it’s used:
If you don’t have tons of topics, I think a quick script using the API would be the easiest way.