When I moved into my house I also inherited a huge bed of orange daylilies in part shade. They bloomed fine, but the first fall I dug the whole thing up, prying apart the roots and tossing about 90% of them. The following years those that remain have bloomed more, bigger, and longer. My guess is they went in about 15-20 years ago, so it was definitely time for help. I’d say dividing is your best bet, but it’s backbreaking (and shovel-breaking) work. They could be turning brown just from lots of afternoon sun? I’ve found that I can also chop down the leaves to about two inches and they will spring back up for me as green and happy as can be.