i think the most significant things that give LotR it's worldfeel is the isolation of the world, the remnants of the past and the subtlety of most of the magic
-isolation: civilisations are few and far between, there are masses of wilderness between any two given locations, it takes days maybe weeks to get from one place to another, and where it's wild there are threats, your little village might have bright lights and warm fires but not too far outside the walls darkness is nipping at your heels, and people know this, most settlements are communities that don't intermingle with each other except in the most fringe interactions, this is especially prominent when you factor the species, most places are mono-species, an elven town, a human village, a dwarven city, the hobbit's shire, making going to other places even more strange and foreign, i don't think having lots of species in a campaign world is inherently bad but having lots of them does contribute IMO to a feeling of 'crowdedness' that is antithetical to the intended tone of a Middle Earth setting, IIRC the fellowship itself or even just aragorn, gimli and legolas travelling together was notable because the species didn't travel together, meeting a dozen of so parties on the road every individual a different shape or colour like your usual group undermines this significance.
-remnants: there are scattered all through the land, for better and worse, bits and pieces of a grander age, they are awe-inspiring to see but the ones in ruin reminding you of how far the world has fallen since then, alot of this is lore, history and stories but quite a few old structures (was it weathertop where the hobbits/strider first fought the ringwraiths?), there is the mines of moria, a once great dwarven city now in decay, home to goblins and demons, there are the two statues of kings flanking the river where frodo leaves the fellowship, the history is often important too, to learn from what came before.
-subtle magic: most magic in Middle Earth low key, yes gandalf or some other spellcaster might throw out some powerful flashy magic here and there but they#re kind of the exceptions that prove the rule, most magic is tied up in more passive manifestations, in learned skills and crafted artefacts, in specific places or times and dates, so when the hobbits do see big displays of magic it's significant to them, related to what i mentioned earlier, species curation is also important for this aspect because some of the DnD species are very conspicuiously magical which isn't beneficial to your Middle Earth goals, the elves also tie into this being an old species who remember the before times, are notable magic and are conspicuously leaving.