I have done this before, but it requires a lot of technical knowledge. If you'd like, I can explain how I did it, but I wouldn't recommend it if you're not familiar with SSH, PHP, and JS. Additionally, it might require you to use a VPS/VDS and make changes to PHP settings.
In short, it involves using a PHP command linked to a cron job to fetch data row by row from an Excel file and send it to Flarum via an API. Of course, during this process, you also need to connect to a database to keep track of the last row you processed.