I can’t find any Legends novels at my library except for a couple of Legacy of the Force, Fate of the Jedi and The Old Republic books, and the Darth Bane trilogy. Also, the Legends stuff I’ve read so far I’ve mostly liked.
The Thrawn trilogy is ok, but I’m not sure you need to read the Legends books before you read the new one. Though it is entirely possible that the new book fits into both canons, judging by its place in the timeline (pre-Rebels). Cause I’m sure Zahn can’t be arsed to care about there being a new canon.
I really doubt they let him just go rogue and write whatever he wanted to haha