I have found the prices have held up very well for smaller LOTR/Hobbit sets like Witch King and Wizard Battle. I had been selling each for £50 up to about a year ago. Once it was obvious Rivendell was coming, I upped them to £70 and they have still sold. Not bad for sets that amazon clearanced at £6 and £5 respectively.
Medium sets are also holding up well. Even the old small Rivendell set - The Council of Elrond - hasn't really been impacted much. I sold my last one of those a few months back for £120. If that hasn't been impacted by a similar subject bigger set, I doubt other sets will.
I haven't tried to sell the bigger ones recently (Helm's Deep, Mines of Moria, Black Gate, etc or the Hobbit sets such as the Bag End, Lonely Mountain or The Battle of Five Armies), although those prices still seem pretty high.
I parted out a lot of my investment sets and of course the minifigures have been doing incredibly well and are the main driving force in the set prices. Figures like the old Aragorn in his brown outfit has kept a reasonable price despite the new version being similar and even the Dimensions version of Gimli has been selling for over a tenner.
I wish I had bought more copies of the Lonely Mountain when they were £50. Smaug has gone crazy now. I thought I did well at the time to buy the set for £50 and sell just Smaug within a week for £50. All those free bricks and minifigures! But these days he is probably £250. Luckily I kept some copies back.
LOTR and Hobbit figures were so cheap for ages where people had bought the sets for the bricks and not wanted the figures, especially duplicates of core characters. Even as late as 2016 I was buying up new figures at under £2 each as people didn't want them. How times have changed.