The assumption about the topology is correct. Building a topic per subscriber is almost an anti-pattern as it couples publishers and subscribers. Having a single topic solves that problem. In terms of performance, there should not be an issue. The broker team would be explicit about recommending against this type of topology if there'd be severe performance implications.
Regarding ordered messages, the Sessions feature is not for publishers. It's for publishers and consumers. It will have a certain drawback such as no scaled-out processing for sessioned messages processing, but that's an acceptable trade-off given you want messages to be ordered.