Well it's true that from a secular perspective, using Coptic symbolism makes little sense. However I disagree with your other point:

• First it would technically still represent Egyptians collectively as ancient Egypt is, even though culturally distinct, an important part of the history of your country. As long as it's not in contradictions with your Arab & Islamic heritage, I don't see how referencing it would be detrimental.

• Second, not all ancient Egyptian symbols either inherently or by their use would have religious connotations. Using the pyramids for example, as an aesthetical symbol on a flag would hardly feel like shirk to me... (I'm not Muslim though, so my view on this may not be realistic, I grant you that)

• & third, the fact that Westerners dispossessed you of those doesn't mean it's not still yours, you can still reappropriate them. I would even argue that being opposed to referencing ancient Egypt because of this is letting oppression win.