It has been a long time since I read about Bible versions so I don't have a specific answer. But there's a book on the topic called If the Foundations Be Destroyed, by Gail Riplinger. It was NIV vs KJV centric but you might be interested in using the same approach to compare KJV vs NKJV.

It was one of my waking up moments back when I was young. It seemed very subversive at the time. The people who were pushing the nkjv were using the reasoning of "I just can't understand all that theeing and thouing! I need something in today's language!", were also the uppity snots that went to "the theater"( in a nasally snotty voice) on Saturday night and watched 4 hours of Shakespeare! Excuse Me! If you can understand Shakespeare you dang sure can understand the KJV Bible! That's about it for me.

Our own language has changed so much since this translation happened, imagine the changes between 2 languages over an even longer period of time and the changes become profound. It would not even be enough to learn ancient languages that all of the original texts were written in, you'd also have to know the way they spoke at the time to fully understand the meaning.

Only for study.

Would be best to be like my son's theological tutor who was often seen just reading the original Greek translation.

Would be nice, but time is limited lol.