Let's go back to what Paul was really saying in Galatians regarding circumcision
Galatians 5
2 Behold, I Paul say unto you, that
if ye be circumcised, Christ shall profit you nothing.
3 For I testify again to every man that is circumcised, that he is a debtor to do the whole law.
4
Christ is become of no effect unto you, whosoever of you are justified by the law; ye are fallen from grace.
And that verse just before Galatians 6:15, which was just before Galatians 6:16 that you used.
15
For in Christ Jesus neither circumcision availeth any thing, nor uncircumcision, but a new creature.
So my point to you was, if those verses I highlight are taken literally,
you will have to conclude that these instructions apply to the Israel of God as well, since you used the NIV translation in the next verse.
So if I am one of those Jewish believers described in Acts 21:20-21, who was brought up by teachers teaching me Genesis 17:14,
I am correct in saying that Paul is teaching me, since I am obviously the Israel of God, that, if I were to circumcise my kid,
- Christ shall profit him nothing.
- Christ is become of no effect unto him
Would that be a reasonable conclusion?