The Dark Legacy books did seem weak but it was the Defenders of Shannara that really finished the series for me.
Maybe he was suffering from the no editor syndrome that can happen when an author gets famous. Not enough push back on bad plot and lack of script tightening. Who knows.
The first "Defenders" was actually based on the (wisely) discarded outline for the original sequel to "Sword", IIRC. I didn't like it all too much, and the third act of the book was really poorly done. The second book, though, with Reyn Frosch, and other "street level" characters, I loved like I haven't loved any of the "Shannara" books since the nineties. - Not quite sure why, just struck a chord with me.
Brooks has actually addressed the editing situation as candidly as he perhaps can: Basically, he personally and understandably gravitates towards more adult themes - the serial killer Ohmsford we get in "Fall", the negative side of the radioactive poisoning that the "Four Lands" are still suffering from, PTSD like with Cogline, or the concept of a sword-wielding monster hunter รก la
Witcher "The Druid's Blade".
...And more often than not, his publishers, who see "Shannara" as a primarily YA-oriented brand, and who insist on him including a couple of core elements in his books - like the "Harry Potter"-ization of the Druids in the Grianne novels, or the ersatz-Daenerys we got in "Fall" - flat out tell him that a certain idea can't be in the books. And so he discards it,
and uses one of their suggestions instead.
This seems to have gotten only marginally better after the MTV series bombed, but has been the mark on Brooks' books since he renewed his contract with DelRey in around 2000. To get him "off the record" on how he
really thinks about being locked in a contract where other folks can essentially dictate him what to put in his books, one of my dreams as a fanboy of fantasy literature.
--- Seems to be modern publishing practice in the US, though: The Dragonlance lawsuit from last year seemed to have a similar problem at its core - "publisher veto", like we're living in the freaking 19th century.