Not as far as I know. JIRA licensing model is not based on concurrent users: https://www.atlassian.com/licensing/purchase-licensing#licensing-1.
Each named user account uses up one licence, although you can manually deactivate and activate users to adjust who counts toward your license limit at any given time. See the section called 'Deactivate a user' here to read about the impacts of doing this: https://confluence.atlassian.com/adminjiracloud/create-edit-or-remove-a-user-794198944.html
If you want to ask Atlassian direct, you should do so via this page: https://www.atlassian.com/company/contact/purchasing-licensing
Hope this helps. Someone may contribute with a better answer.
You've pretty much covered it. I'm not sure what is meant by "floating" licence, but the model you've described there is the only one Atlassian do really. It changes a bit in some ways for very large servers (data centre), but Cloud and Server licencing is as you wrote.
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