This is a fascinating passage, Rog, when you consider that Peter's primary audience here were messianic Jews. Peter is telling those believing Jews that at one time they were not "a people" (i.e. God's people), even though God chose an entire nation with which to enter into a covenant relationship. This goes to the heart of what Paul taught in Rom 9 when he explains how there are two kinds of Jews -- those according to the flesh (physical descendants of Abraham) and those according to the promise.
I don't mean to be argumentative, nor do I want to be too dogmatic, and I understand what you're saying, but I'm not so sure it was intended that 1 Pet 2:9-10, to be only addressed to the messianic Jews, given as Peter salutation to them was as being "strangers" in 1 Pet 1:1, and from what I could determine from scripture, the term "stranger" is often used to describe the non-Jews who joined Israel - though in this case joined to the spiritual Jews of the Israel of God, not earthly Jews of earthly Israel(see Eph 2:11, 12 below); that is, they, had been strangers to the new covenant (which we all were at one time or another). So, I also believe the Gentiles, as well as the Jews, were the grouping of 1 Pet 2:9 -10. I think Paul's teaching was that God's election was/is the sole, predominant, and determinative grouping supplanting all other earthly associations, with those He elected comprising the "hast redeemed us to God by thy blood out of every kindred, and tongue, and people, and nation; " Rev 5:9, to include the elect earthly Jews.
Regarding Rom 9, I think that beginning at 9:6, Paul changes (albeit subtly) to speaking about spiritual Jews, not earthly Jews - see Gal 3:29 below.
Therefore, the election, and only the election, became the people of God irrespective of earthly linage.
Not saying that I understand it all perfectly - how it all fits together (so I am open to correction) - nor did I explain it the best way possible, so hope it makes sense.
[Exo 12:48 KJV] 48
And when a stranger shall sojourn with thee, and will keep the passover to the LORD, let all his males be circumcised, and then let him come near and keep it; and he shall be as one that is born in the land: for no uncircumcised person shall eat thereof.
[1Pe 1:1-2 KJV]
1 Peter, an apostle of Jesus Christ, to the
strangers scattered throughout Pontus, Galatia, Cappadocia, Asia, and Bithynia,
2
Elect according to the foreknowledge of God the Father, through sanctification of the Spirit, unto obedience and sprinkling of the blood of Jesus Christ: Grace unto you, and peace, be multiplied.
[1Pe 2:9-10 KJV]
9 But ye [are] a chosen generation, a royal priesthood, an holy nation, a peculiar people; that ye should shew forth the praises of him who hath called you out of darkness into his marvellous light:
10 Which in time past [were] not a people, but [are]
now the people of God: which had not obtained mercy, but now have obtained mercy.
[Eph 2:11,12-13 KJV]
11 Wherefore remember, that ye [being] in time past Gentiles in the flesh, who are called Uncircumcision
by that which is called the Circumcision in the flesh made by hands;
12 That at that time ye were without Christ, being aliens from the commonwealth of Israel, and
strangers from the covenants of promise, having no hope, and without God in the world:
13 But now in Christ Jesus ye who sometimes
were far off are made nigh by the blood of Christ.
[Eph 2:19 KJV] 19
Now therefore ye are no more strangers and foreigners, but fellowcitizens with the saints, and of the household of God;
[Col 3:11 KJV] 11
Where there is neither Greek nor Jew, circumcision nor uncircumcision, Barbarian, Scythian, bond [nor] free:
but Christ [is] all, and in all.
[Gal 3:28,29 KJV]
28 There is neither Jew nor Greek, there is neither bond nor free, there is neither male nor female:
for ye are all one in Christ Jesus.
29
And if ye [be] Christ's, then are ye Abraham's seed, and heirs according to the promise.
[Gal 6:15 KJV] 15 For
in Christ Jesus neither circumcision availeth any thing, nor uncircumcision, but a new creature.