A "nuclear option" I've seen in the wild is Queue-It. Puts your entire flow behind a queue that's user-visible, with a waiting list page. Then presumably your own site's logic doesn't do any queuing.
How do they handle multi-tenancy for their users? Any post on that?
I don't know, but the requirements are different. They manage a single huge queue for your website (or part of it), and your own backend is expected to be able to handle some trivially small number of concurrent users. The most common use case is highly contentious online ticket sales, where users might even wait an hour in line. Latency isn't much of a concern.