I don't think many people buy third-party content. I haven't bought the books.
I don't know what you mean by "more serious" but maybe you mean harder? If someone is playing a game that is "true" to LotR then D&D 5e won't actually fit. At low levels PCs have very few hit points and few Hit Dice (poor recovery). This is bad because PCs can turtle after getting mauled by an encounter. (Adventuring while half dead is foolish, but not adventuring to save the world is also foolish.)
I know of a group that did a pretty good job of this with 4e (high starting hit points, but slow inflation, and very good out of combat non-magical healing) at low level. They made Aragorn a warlord because the term ranger means different things for different players. :)
I've seen blogs saying you could do LotR at very low levels (back in 3e) but I don't agree. Gimli and Legolas can each kill almost two dozen opponents in one lengthy encounter. Aragorn himself has done quite a few things prior to LotR such as defeating the third Numenorean kingdom. The halflings seem to have very high AC (they rarely get hit, and then that gets turned by their magic armor). And trolls get killed by sunlight (puzzle bosses that are overpowered if they don't figure out their massive weak spot).
Come up with better non-magical healing. No need for better in-combat healing, but perhaps more Hit Dice. Avoid starting at 1st-level (but LotR PCs probably don't reach high levels either). Limit classes - no obvious magic for any classes that have spellcasting. (Something like the ranger's aiming spell would be fine. The only wizards or clerics would be elves who never leave their lands.)
Long waits between rests are alright. The LotR characters sometimes fought in (and against) entire armies, or made desperate forced marches through mountains.