Just make it so the max players you can have in a game is 10, and once the boat is full with 10 those 10 players are sent to the island.
The remaining players, as long as it is 5, can wait for the next game to start in 30 seconds. Make it so more than one game can be run at the same time.
Being able to join the game while one is currently going would be useful, but it could also throw the whole game off because a lot of the time players don't know and/or don't care if that game is 'farming' or not and they're just there for fast games.
Whole lot of work for a potentially increased chance at failing the minigame. This would be a disaster.
I run games with 5-6 players all the time. This doesn't increase or decrease the chance of failing the minigame at all.
I prefer running games with lower players anyways, much more smooth forward going and people actually understand when I explain how to 'farm'.
The more people the easier it is to kill the portal which makes it easier to "fail" the game when players are trying to farm.
The less people means the less people you have to protect the void from dying, which also makes it easier to fail them game. This is the only way I see it 'failing'
Adding in another island for more than one game to run at a time doesn't add any extra scarce of failing the game, just allows players to be able to enter games quicker as they won't have to wait minutes for one game to end because the current game running is farming.
We would be failing by not implementing this which would be allowing players to go in and out of games quicker which would could potentially make the game more popular because you don't have to stand in a boat for 5 minutes waiting for a game to start or you don't have to go through a game in 2 minutes getting 75 tokens because the people you're playing with don't know how to farm.