So you do not see that Sarah is being juxtaposed against Hagar? or that Sarah is being likened to Yerushalem of above? or that Yerushalem of above is being likened to the correct understanding of the two covenants which he is speaking about in this passage?
The answer is right there in what you quoted:
Gal 4:29 But as then he that was born after the flesh persecuted him that was born after the Spirit, even so it is now.
It is the same juxtaposition of the flesh -vs- the Spirit which Paul speaks so much about in his writings. Those who view the covenant according to the flesh and the physical mindset either reject it entirely, as many here do, or they accept it according to the flesh and the mind of the flesh and end up doing the "works of the law" which cannot save them.
Those who view the covenant in the new-renewed way of understanding walk in the same Torah but according to the new-renewed Spirit also foretold in the Prophets, (Ezekiel 11:19-20, Ezekiel 36:26-27), by way of the Testimony of the Messiah in the Gospel accounts.
The two are the same covenant but viewed through two different sets of eyes. The one who sees all things according to the eyes and mind of the natural and physical minded old man nature is the one who abides in Hagar because he or she walks according to the flesh and the mind of the flesh.
The one who is renewed in heart and mind, and made into a new creature, goes back into the womb of his or her mother, (Yerushalem of above, our mother, (covenant)), and begins the process of relearning all things according to the doctrine, teachings, and Testimony of the Messiah which is freely provided to us by the grace of Elohim in the Gospel accounts.
If Nicodemus the Teacher of Yisrael can do it, (John 19:39), and if Paul a Pharisee of Pharisees can do it, (Galatians 1:14-16), and if it is also true of even Timothy, (2 Timothy 3:15), then no doubt we can all do the same. Moreover this is not something new, for the Prophets Yeshayah and Yirmeyah also speak of these things, (Isaiah 44:2-24, Isaiah 49:1, Jeremiah 1:5).
Moreover the answer to Nicodemus in John 3:4-5 is literally "Amen, amen", and that is a double affirmation: the first amen literally means "So be it", and is therefore the answer to his question in John 3:4. The second amen, (verily or truly), pertains to what follows.
All one needs to do is actually understand and believe what Paul says in the Galatians passage you are quoting from: Yerushalem of above is indeed your mother, (covenant, because it is an allegory of the two covenants), and therefore you can indeed "go back into the womb of your mother" and relearn everything according to the new-renewed way that the Messiah has shown to us by his Testimony, doctrine, parables, proverbs, sayings, idioms, and teachings in the Gospel accounts. Moreover Paul teaches the commandments of the Master, just as he says in several places, (1 Corinthians 14:37, 1 Thessalonians 4:1-2).
PS: Since this isn't the thread topic I'll leave it here.
Nice chatting.