Well, you're not very wrong. Certainly you have the Istari's role perfectly correct. My only caveat with your comments is that I believe Tolkien did not oppose the use of magic per se. Rather, he opposed the use of magic to dominate either men or nature. Both Sauron and Saruman are guilty of these sins. Anyone who took the ring and tried to use it "for good" would also be guilty of this sin.
The contrast between Gandalf and Saruman is most instructive. Saruman settles in at the fortress of Isengard and eventually tries to become a lord of men. That's where he goes wrong. Gandalf, however, never tries to have any personal power. Rather, Gandalf puts fire in the hearts of free people so that they themselves may rise up and fight against evil. Gandalf's use of magic in this effort is not improper because he gains nothing more than the diminution of the forces of evil in the world. This is a gain, of course, but it is not his personal gain.