You can block spawnpoints with the runway object. So if you custom runway is over the default spawnpoints they are not available anymore. Same with the selfmade spawnpoints. They must not be covered by runway.
If all your spawnpoints are blocked by objects (even near objects like crate too close) or used by a player who just spawned, the next plane will be spawned along the circle with the direction ot the centre of you airfield. So you can artifically create a half circle of spawnpoints, by blocking your default spawnpoints wiht a wide runway and putting the centre of the spawn are cirecle in a way, that only a curve of the cirle lies outside the runway object. Nowr you planes should spawn along this curve (reaching beyond the runway box).
Like this old picture:
1. pic: the square box of the object airfield blocks the spawnpoints and the centre of the spawnarea is totally covered by the airfield box. Now the circle is adjusted that it just touches the box at the border. All planes now spawn along the border, where the circle touches the box border.
2.pic: looks like this below
