What you seem to be running into here is the change in philosophy from the schoolmen to the moderns. In the course of that transition, several philosophical concepts were replaced with other concepts using the same name. For example, definitions used to be viewed as a part of understanding the thing defined. It revealed a truth about the thing defined. A definition could be wrong in the sense that it did not properly bring out the essence of the thing. In modern philosophy, by contrast, a definition is just a matter of language. It is a stipulation of how you intend to use a particular word. It can't really be right or wrong, only conventional or unconventional (or it can fail to pick out anything, but that's a whole topic of its own).
Similarly, it used to be that a thing was in some sense constituted of its properties. An object was viewed as a bit of formless matter that properties glommed onto, creating a form and producing a composite object. An object was an aggregate of matter and form. But an aggregate can be disaggregated, which was not viewed as a proper attribute of God, so God had to be a sort of object that was not an aggregate, could not be disaggregated.
In modern philosophy, a property is just the sort of thing you are thinking of, a description of how a thing is perceived, and not an integral part of the composition of an object. Another way to say it is this: in modern philosophy, a property is generally viewed as a sort of mutual feature of the observed and observer rather than a feature of the observed alone. Of course there is a lot more to say about this, and materialism arguably requires some sort of essentialism at some level of reality, but in general, properties are not viewed as constitutive.
So, the arguments about the simplicity of God don't really makes sense when you are using the modern notion properties. You have to understand their arguments in their terms. Here is a SEP article that tries to explain their arguments.
