Hello @Andrew Wilson,
welcome to this moderated Azure community forum.
Yes, as seen in the documentation, there are limitations to both the number of Topic spaces, and Topic templates:
But a topic template is not limited to one topic:
You can add multiple topics.
As an alternative, you could try to use a wildcard to support a whole group of sub topics using the '#' or '+' together with the topic separators.
If this doesn't help, create a support ticket and explain to Microsoft why you need extra topics. They are always interested in real-world use cases.
If the response helped, do "Accept Answer". If it doesn't work, please let us know the progress. All community members with similar issues will benefit by doing so. Your contribution is highly appreciated.