***the slave of sin is crucified with Christ, ***
On the cross, not in the water. Why else would Paul state “I bear in my body the marks of the Lord Jesus”. He certainly did not get them when he got baptized.
***He says that to be “of Christ” requires being baptized into Christ.***
Into Christ by the action of the Holy Spirit. Again not water.
When we go into the water, our sins have already been remitted on the Cross through the death of Christ. The rest is merely symbolic.