Can you try publishing a post without using the custom template now? It may be that all that needed to be done was to add https://www.youtube.com/embed to your Discourse allowed iframes site setting.
Which editor are you using on your WordPress site, the old Classic Editor, or the new Block Editor? Also, are you publishing the full post to Discourse, or just an excerpt?
This could be done by writing a small script to publish posts from a CSV file.