But you are correct in that one of Tolkien's great themes is the corruption of power. Not just magic, but any power can corrupt in Middle-Earth and the wise know not to take for granted their power and not to use it too readily.
Well, Aragorn had power and used it without being corrupt. Ditto Theoden. Denethor had a problem, but he lacked humility and attempted to challenge Sauron with one of Sauron's favorite tools. I don't think Tolkein's theme was power corrupting but the necessary humility for handling power. And one element of humility is that magic isn't for human beings. At least, that's what I got out of it without reading the appendices or The Silmirilion.
Shalom.
Another theme from the books is that "the great" might not have the greatest roles, and stupendous deeds may have to be done by the least great of the creatures, like soft, complacent Hobbits. In fact, when the greatest of The Great get too big for their britches, it's only "the little" than can take them down.
Yet another theme expounded in the books is that of passing, loss, and diminishment. And even the destruction of Sauron brings about the slow fading of all the creatures except Men themselves. Yet all the free creatures of Middle Earth realize that they have to pay that price, in order to free the universe from Sauron's evil. It takes a special kind of courage to fight a war where the only outcomes are your eternal enslavement to evil, or the slow fading of your kind, while the far less noble Man gradually ascends. There is a strong theme of sadness and loss, even in victory, present in the work.
I've read these books many times, but manage to notice different themes each time I read them. This time around, I'm noticing the feeling of cold, misery, fear, and doubt that dogs the characters when they're not resting in some blessed oasis like Rivendell, or up to their armpits in orcs. It's a common feeling for anyone who's been in the military, and something Tolkien remembers from WW1, while he writes to his son, in WW2.