Is the limit of 23 list topics a "hard" limit, and what are the alternatives if I need more topics?

Yes, it is a hard limit and can't be raised.

If you need more topics, the only alternatives are:

  • Use multiple lists and/or better categorize your topics
  • Switch from using topics on a single list to using a combination of a super-lists and sub-lists

The fact is that if you need more than 23 topics, you probably need to rethink your topic strategy.