There may be a way to do it using your spawn method as it is. But you could always do it yourself.
Before you spawn an actor, use Sphere Overlap Actors to probe the spawn location. If you get overlaps then adjust the spawn location yourself so you can ignore the Z component. There is also Box Overlap Actors if you want to get a nice tight fit for box-shaped actors. And there is Capsule Overlap Actors to get a tighter fit for capsule-shaped actors (like humanoid characters).
Once you get a good location you can spawn them as “Always Spawn, Ignore Collisions” and it will spawn exactly where you tell it to (i.e. on your desired plane).
And you may want to test an area that is a bit larger than the spawned actor. That is, if things will be moving around during spawns (which they usually are). The larger area will account for the movement so you can be sure that by the time the actor is fully spawned, nothing has moved into its actual spawn area.
Hope that helps if you can’t find a built-in way. Good luck!