"Load until X is available" is what you are looking for. You can specify both a minimum and maximum amount for this order, if there aren't enough of the mineral type to pick up, the ship will wait until there is.
The only problem is that this order by mineral type - I think you can maybe use the minimum amount setting on load minerals in order to get what you want. Just set it to your cargo space / 2.5 and the ship/fleet should wait until its full.