Is there an external material world ? You seem to be attacking a straw man version of idealism.
Berkeley is the most famous and best idealist. And he argued that the external world that our sensations give us some awareness of must itself be made of sensations. He got to this conclusion in the following way. First, he noted that our sensations can only be said to be giving us some awareness of an external world if they in some way resemble it. If our sensations in no way resemble the world they're supposed to be telling us about, how do they give us any awareness of it?
Next, he held that it was self-evident to reason that a sensation can only resemble another sensation. Sounds are like sounds, smells like smells, textures like textures and so on.
From this it follows that the world our sensations give us some awareness of must itself be made of sensations, for it is only such a place that they could possibly tell us about.
Next he held that it was also self-evident that sensations are essentially sensed. That is, they cannot exist unsensed.
Next, sensations are always and everywhere sensed by a mind of some kind. For any sensation, there is a sensor, and the sensor is a mind.
Thus the sensations constitutive of the external world are being sensed and by a mind, for only a mind can sense things.
And as the external world is unified - there's 'the' sensible world - the external sensible world is the sensational activity of a single mind. Not yours or mine, but another.
It is noteworthy that many contemporary critics of Berkeley's idealism also attack straw man versions of it. For instance, many critics seem to think that Berkeley was arguing that the sensible world exists in 'our' minds. That was not his view. The external world is every bit as external on his view as it is on a materialist view. It is not its location that he is disputing, but its composition.