The summon command syntax is:
/summon <EntityName> [x] [y] [z] [dataTag]

this suggests that if I only include the [x] and not the [y] [z] [dataTag], the command should operate differently to just /summon <EntityName>.

This is not the case though, if you wish to change only the x you would need to enter the y and z.
So I would recommend one of two options:

• Change the syntax of the 3 coordinates to that of the /spawnpoint 3 coordinates like: [<x> <y> <z>], suggesting the requirement of all three coordinates together as an argument or none of the three.

• Keep the current syntax, but if the player only enters, for example, ~5, for [x], assume the y and z are relative (~) and summon the entity 5 blocks in the positive x direction (currently, /summon Creeper ~5 just summons the Creeper where the player is).