I would recommend rewriting your original post, I thought it was what the above people thought until I saw you tell them otherwise.
I don't like this idea. Donors pay for the servers, and they deserve the right to join servers when they want to. It's kind of unfair to force them to join servers in the last 30 seconds. Then they can't do any of the parkours if they want to during the lobby, and they stand no chance of getting that diamond armour if they want to do that too.
It also makes it a lot harder for donors to play with their friends, because they only have a 30 second time period where they can join.
In addition, having donors only join in the last 30 seconds would give non-donors a false sense of security for the first 2 minutes of the lobby. And then it would be more painful when that flood of donors comes in during the last 30 seconds.
Also, 48 player servers have 4 minute lobbies. That gives them even less of a chance of joining.
And what about those big events on eu9001? Still only the last 30 seconds?
I understand your reason for wanting this, but I think it just makes it unfair and limits the donors too much.