Enable Cloudflare Waiting Rooms mid-event

I website I manage has planned promotional events every so many months that create significant spikes in their visitor numbers for a 6 hours or less. During 4 of those 6 hours the server becomes slow and sometimes unresponsive.

They do not want to upgrade the server or move to flexible cloud servers so I was going to suggest upgrading to Business plan to enable waiting rooms.

Could it be configured in advance and turned on mid-event as I am monitoring server load whilst simultaneously monitoring the numbers of users in the waiting room dashboard? I understand that you can set number of active users permitted etc in advance so that waiting rooms comes on automatically however I have no data on the number of visitors that would cause the high server load.

So my question is: Can it be switched on mid-event or is there any other suggestions on how to handle it?