So, taking up the Cross is optional?
No, taking up the cross is the result - it all starts with justification (which the law could never provide).
Without justification we are lost - and we cannot achieve (nor add too) justification.
It’s not optional. It’s a matter of who does the works, and who should get the credit for them.
“I am the vine, ye are the branches: He that abideth in me, and I in him, the same bringeth forth much fruit: for without me ye can do nothing.” John 15:5
“I am crucified with Christ: nevertheless I live; yet not I, but Christ lives in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me.” Galatians 2:20