Yes, that could well be said. Though of course others would disagree.
But as far as a "character" and a "central or key role" only
Frodo becomes the possesser of
the ring, the bearer of the ring, the owner of the ring claiming it for his own, and the loser of the ring. Not unlike the unfurtunate owner of
Anduril who possessed it prior to he, but unfortunately died at its loss. None other fulfills all those functions in LoTR.
The Ring itself is, in my view, the central "thing" in LoTR.
Sauron's contant pursuit of it.
Gandalf's protection of it. Sauron's lust for it. The obsession of all "the great" with it. The entire work is focused on "the ring."
But here's the thing... is it really 'the ring' they're focused on?
Or is it something else. Something the Ring only represents.
And something Frodo, and Gandalf, and the Free
Peoples, and
Melkor, and Sauron, and the
Orcs, and Sauron, and Lobelia
Sackville-Baggins only represent.
Yes,
Tolkien said at one point "he really just wanted to write a really long story." He also said "he wanted to create a mythology for his country, something other countries had, but England didn't." He also said in one interview "the central pin of his story - in fact of all good stories - is death, isn't it?" And, he said it was "primarily a religions and Christian work."
Tolkien said, at different times, very different things about his work.
Why? I believe it's because his work was bigger than himself, and in the end even he didn't quite know how to relate to it fully, any more. 'Tis why
Bombadil is in there. Bombadil was a very important element in Tolkien's "understanding of all things." And frankly, the existence of a Bombadil serves as a foundation - an underpinning - for all he ever did.
So we can find, and hold different opinions, over "key characters", "key things", "key scenes", "pivotal points", "meanings", etc. ad infinitum. And we will always have differences, and even our own thinking will change from season to season.
Because Tolkien's work comes closer to "Truth" than any other story ever written.
And while "The Truth is out there" - none of us really know what that "truth" is. Nor can we, in this life.