I'm not so sure Tolkien could be fitted so easily in the context of an evolution of approach in modern and pre-modern literature (with his predecessors, I mean. It's of course much easier to place him in such a continuum looking at his influence on others later)

In his use of symbolism, Tolkien was as "unmodern" and "uncontemporary" as could be IMO. His symbolic language is essentially borrowed from mythology. Obviously some of his recurring motifs had deep personal signifiance to him, but they remain extremely easy to decipher, as he barely strayed off the beaten path (in doubt, we even know which domains in mythology to look into and which to exclude to precise meanings or paths to decipher him).

I wouldn't see his use of symbolism a sign of his literary talent or his imagination so much as a sign of his scholarship and the profound intimacy he had with the texts and literary genres he took his inspiration from (which probably explains in turn why Tolkien's symbolism has not been the focus of scholars the way for instance Joyce's has been - including a Jungian essay by Joseph Campbell about Finnegan's Wake, as it happens. I doubt he would have thought LOTR a worthy subject of study, except from the angle of its mass appeal being rooted in its use of mythology archetypes, as he did with Star Wars). With the publication of the Silmarillion that gave the last few "keys" to the symbolism in LOTR it became fairly transparent.