Something like Inngest's multi-tenancy aware flow control should be table stakes for most complex products now: https://www.inngest.com/docs/guides/flow-control.

Huge disclaimer here: I'm one of the founders. You should most definitely be able to set concurrency levels, throttling, rate limiting, etc. per your own tenant, in code, without having to mess around with creating independent queues or streams for users, and without managing state. We let you do that.