Ever read Acts 1-7?
It was working, at least initially.
Agreed.
This is literally the plot of the Bible.
Agreed.
Israel was always meant to be a spokes-nation to the world. God had spokesmen (prophets), and tried to scale up. Acts shows that it didn't work.
Agreed.
So God went to plan B, a mystery kept secret from the foundation of the world,
Agreed, but would it have remained a mystery had the first plan worked? If it was planned that Gentiles would reap the benefits of the grace of Christ from the very beginning, and it seems like it was, God could have brought it to pass through either plan. Because the first planned failed, the second plan was necessary, but that doesn't mean the objective was different, only that it was to be achieved through a different plan.
using one person to go to all nations.
I don't think that was the original plan B, since Paul was never alone in his journeys.
New way?
Try "mystery kept secret from the foundation of the world."
This is the plot twist of the Bible.
"You must be circumcised" and "you don't have to circumcise" are the same message?
They are not. But "you must be circumcised" was not the gospel, ever, as far as I can tell.
Nowhere in the gospels is Christ's death burial and resurrection openly preached.
No, because it would not result in God's plan for Jesus' death burial and resurrection. In fact, death burial and resurrection are not the gospel--they are only good news in the context of salvation, as you affirm in your next sentence.
And prior to Paul's conversion, Christ's crucifixion is preached as a condemnation against the Jews, not as the core tenet of salvation. (cf, Acts 1-7).
But that's the core claim of Paul's message and subsequently Christianity as a whole, that Christ was raised to bring life to the dead, not to the remission of sins (which is something only found in the Gospels, Acts 2, in Acts 10:43 in the context of Peter and Cornelius, and the epistle to the Hebrews).
But if sins are what is causing the death that needs to be rectified, then it is the same thing, similar to when Jesus offered the two phrases "take up your pallet and walk" vs "your sins are forgiven you." (From Mark 2:9)
In fact, Paul speaks of the same thing, using the same word, 4 times, only it is translated "forgiveness" instead of "remission" (I don't know why):
"Therefore let it be known to you, brethren, that through this Man is preached to you the
forgiveness of sins; [Act 13:38 NKJV]
'to open their eyes, [in order] to turn [them] from darkness to light, and [from] the power of Satan to God, that they may receive
forgiveness of sins and an inheritance among those who are sanctified by faith in Me.' [Act 26:18 NKJV]
In Him we have redemption through His blood, the
forgiveness of sins, according to the riches of His grace [Eph 1:7 NKJV]
in whom we have redemption through His blood, the
forgiveness of sins. [Col 1:14 NKJV]
But pretending it is a message distinctive when it is merely translated in a synonymous way is disingenuous.