I generally don't spring for Director's Cuts unless the original film is one I felt was considerably modified or edited down for theatrical release. I'm not a fan of more material just for the sake of more material, (though I tend to give book adaptations a lot of leeway here).
In the case of LotR, I think the first two movies are quite improved by the Extended Cuts (especially The Two Towers, which I did not like in its theatrical version). Much of what's added in deepens the characterization or allows the film a bit more room to breathe, and these are movies that exult in the vastness of their scope, so that all works for me. I like seeing book material adapted well.
Return of the King is the one that seems really perfunctory; half the time it feels like they were just inserting sequences of no particular quality in order to hit a new record run time, and I think that movie's both narratively and thematically more satisfying in its original incarnation.
From what I've read, the additional material in Doctor Sleep mostly serves to strengthen the film's existing themes rather than alter the narrative in any meaningful way. There are several sequences in the theatrical version that feel as though they were cut short for time (the movie ran over 2.5 hours!), and I think giving them some room to stretch their legs is not a bad idea. It's a somewhat languorous movie, and I felt that pacing was valuable. The theatrical cut felt a little rushed in the back third, and I'm hoping it feels less choppy this time around.
I read Mike Flanagan refers to this as his 'personal cut,' and I liked what he did with the original version enough to give him benefit of the doubt.