I think it might just be an training/experience issue rather than the fault of the product.
Have you done any specific colour courses with Wella? Some wholesalers run short half day courses that aren’t expensive and you get to speak to experienced trainers and swap ideas with fellow participants.
Personally, I’m not keen on NXT as it’s a cheaper brand and I think the quality just isn’t as good across the range. However, I’m not salon based and do like to use a few different brands depending on what’s required. For instance, I use illumina for a few of my blondes. I originally trained with Goldwell and still use them occasionally. L’Oréal have some lovely shades. I also really like Schwarzkopf Blondme bleach.