My point is that Paul is alluding to OT passages that show the Jews were chosen as a people and they were predestined to adoption as a people.
No, the Jews were not predestined to adoption through Christ.
Let's look at the text.
"he predestined us for adoption as sons
through Jesus Christ, according to the purpose of his will"
This adoption though predestined, did not take place until the coming of the Messiah, so it cannot refer to just the Jews.
God called Israel His Son, and the "Beloved" that Paul speaks of is Israel, not Jesus.
Beloved is a term that Paul uses very often to refer to all believers.
To all those in Rome who are
loved by God and called to be saints: Grace to you and peace from God our Father and the Lord Jesus Christ. Romans 1:7 (ESV) [Note: Same Greek Word]
Beloved, never avenge yourselves, but leave it to the wrath of God, for it is written, “Vengeance is mine, I will repay, says the Lord.” Romans 12:19 (ESV)
Therefore,
my beloved, flee from idolatry. 1 Corinthians 10:14 (ESV)
Therefore be imitators of God, as
beloved children. Ephesians 5:1 (ESV)
Therefore,
my beloved, as you have always obeyed, so now, not only as in my presence but much more in my absence, work out your own salvation with fear and trembling, Philippians 2:12 (ESV)
Beloved, are those people who are loved by God, which is certainly not excluded to Israelites, but to all those in the Messiah.
If you read the other link it goes into greater detail and shows how the Greek grammar excludes certain interpretations that many Christians try to apply to this passage.
While it is important to note the differences, which he makes clear in verses 11-14, this is not so for verses 3-10.
The first person plural pronoun simply means that it is the author plus someone else. It could mean Paul plus Joe, or it could be Paul plus everyone in the world. It does not have an definitive quality to it so as to exclude a certain interpretation except that it could only be referring to Paul only, that is not possible.