For the most part, I agree with you. There is a sense in which one can say mathematics exists only when there is a mind to think about mathematics. There is some physical construct of the numeral 1 in my mind, and so it exists in that sense.
Can't you say the same of any descriptor? "Blue" only exists in the mind as well. And yet there is something that exists even when the mental concept "blue" doesn't exist (light at specific wavelengths). At that point it becomes a matter of agreement in the way we use our language. If we agree "blue" refers not to a mental concept, but only to specific wavelengths of light, then we can say blue exists.
It seems it then becomes a matter of what satisfies a particular person. Electrons have no known subparticles, and so some may be satisfied to say an electron is the fundamental thing that exists and stop there. But electrons can be made from photons and annihiliated (with a positron) to emit photons. As such, others are not satisfied that the electron is the fundamental thing and want to go a bit further down the infinite chain of turtles.
So, given the speed of those photons in a vacuum is ~3E8 m/s, and that photons don't have a rest state, is there a sense in which the existence of a photon is dependent upon a thing we call 3E8 m/s?