I’ve always thought that it was in striking the rock TWICE, rather than just once. A lack of trust of God on the part of Moses.
I see striking the Rock as a Type of the crucifixion of Christ, who was broken for us.
Moses struck the Rock twice, which I see as ‘crucifying the Son of God afresh (Heb 6:6, below)
This would be a lot more serious than just Moses’ pride, disgust with the Hebrews, etc., and would explain the more serious repercussions.
Hebrews 6:
4 For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost,
5 And have tasted the good word of God, and the powers of the world to come,
6 If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.