This figure of 5000 is for peak concurrency i.e. do you think at any point in your app, you will more than 5000 users logged in at the same time.
If so, neither the free nor the pay per use model I believe works for you in terms of pricing. You might need to go for peak concurrency usage option.
Contact us in details about your use case if you feel you will have more than 5000 concurrent users.
As per limits, theoretically, you can use s2s APIs to receive message from one room and route that to another if you feel all users let's say (8000) at the same time needs to be broadcasted the message.
Hope this helps
Thanks
Hironmay Basu