NMIXX weren't rushed, in terms of Training, Sullyoon had the least amount just over the year while the rest were over 3 years.
When TWICE debut, Mina had trained for over a year while the rest of the members were over 3 years where Momo, Sana, Dahyun, Chaeyoung and Tzuyu all joined in the same year, 2012
Itzy are similar, Lia training for a few years while the other members trained for over 3 years.
The core members of each group were likely grouped as trainees because that's generally how JYPE Trainee System works.
The other indicator that NMIXX wasn't rushed is that they debut from Division 4 (Studio J) whom were just managing XDinary Heroes as Day 6 were Serving.
Division 2 has Itzy, Division 3 has TWICE, JYP Japan has NiziU, so NMIXX could have debut under Division 1 or Studio J. I also bet that NMIXX would have had different music if they were under Division 1 because it's Studio J that decides what Music NMIXX gets so it's typically more Non-Idol in Structure.
