So if Charles I was collecting, in a sense, to self-aggrandise and to make a statement about himself, whereas George III was collecting because he thought that's what a king should do, because he loved art?

Yes. He would never have commissioned an image of himself as flamboyant and theatrical as this. I think in selecting this painting to hang at Buckingham House, he would have been thinking much more in terms of the reputation of Van Dyck as a kind of the founding father of English painting and as the supreme exponent of elegance.

模仿部分~

George III saw it as part of his royal duty to promote the arts, and in 1768 he founded the Royal Academy, to encourage the work of British artists.