That Acts 21 passage has the answers you are looking for, for some reason, you refuse to consider that passage
18 And the day following Paul went in with us unto James; and all the elders were present.
20 And when they heard it, they glorified the Lord, and said unto him, Thou seest, brother, how many thousands of Jews there are which believe; and they are all zealous of the law:
24 Them take, and purify thyself with them, and be at charges with them, that they may shave their heads: and all may know that those things, whereof they were informed concerning thee, are nothing; but that thou thyself also walkest orderly, and keepest the law.
25 As touching the Gentiles which believe, we have written and concluded that they observe no such thing, save only that they keep themselves from things offered to idols, and from blood, and from strangled, and from fornication.
And if that is not enough, here is what Paul stated of the reputation of Ananias in Acts 22, which is already so many years after the cross
12 And one Ananias, a devout man according to the law, having a good report of all the Jews which dwelt there,
You are still unwilling to accept that Jews who believed in Christ as their Messiah during the time in Acts, still needed to follow the Law of Moses?
Romans 2:28
For he is not a Jew, which is one outwardly; neither
is that circumcision, which is outward in the flesh:
Romans 2:29
But he
is a Jew, which is one inwardly; and circumcision
is that of the heart, in the spirit,
and not in the letter; whose praise
is not of men, but of God.
Galalatians 5:1
Stand fast therefore in the liberty wherewith Christ hath made us free, and be not entangled again with the yoke of bondage.
Galatians 5:2
Behold, I Paul say unto you, that if ye be circumcised, Christ shall profit you nothing.
Galatians 5:3
For I testify again to every man that is circumcised, that he is a debtor to do the whole law.
Galatians 5:4
Christ is become of no effect unto you, whosoever of you are justified by the law; ye are fallen from grace.
Galatians 5:5
For we through the Spirit wait for the hope of righteousness by faith.
Galatians 5:6
For in Jesus Christ neither circumcision availeth any thing, nor uncircumcision; but faith which worketh by love.
Paul’s stance on circumcision is clear from the passages above.Also Peter is aware from the incident with Cornelius that circumcision is not a requirement for salvation.(Acts 11:1-8)
Paul also received confirmation that he received the same message as those who were apostles before him.(Galatians 2:1-10)Notice that Titus was not compelled to be circumcised.The charge was to go to different groups not with a different message.This was confirmed by the actions of Paul who preached to both Jews and Gentiles as he brought the message of salvation.(Acts 20:18-21;13:39-46;14:1-4;18:1-6)
In contrast to how Paul treated with Titus we see Paul having Timothy being circumcised while preaching the gospel message.
Acts 16:1
Then came he to Derbe and Lystra: and, behold, a certain disciple was there, named Timotheus, the son of a certain woman, which was a Jewess, and believed; but his father
was a Greek:
Acts16:2
Which was well reported of by the brethren that were at Lystra and Iconium.
Acts 16:3
Him would Paul have to go forth with him; and took and circumcised him because of the Jews which were in those quarters: for they knew all that his father was a Greek.
Acts 16:4
And as they went through the cities, they delivered them the decrees for to keep, that were ordained of the apostles and elders which were at Jerusalem.
Acts 16:5
And so were the churches established in the faith, and increased in number daily.
Is Paul going back to the requirements of the law?No.He is practicing the principle he expounded in the following passage:
1Corinthians 9:19
For though I be free from all
men, yet have I made myself servant unto all, that I might gain the more.
1Corinthians 9:20
And unto the Jews I became as a Jew, that I might gain the Jews; to them that are under the law, as under the law, that I might gain them that are under the law;
1Corinthians 9:21
To them that are without law, as without law, (being not without law to God, but under the law to Christ,) that I might gain them that are without law.
1Corinthians 9:22
To the weak became I as weak, that I might gain the weak: I am made all things to all
men, that I might by all means save some.
1Corinthians 9:23
And this I do for the gospel's sake, that I might be partaker thereof with
you.
Hence the actions of the Acts 21 passage you quote.Paul could not condemn those he wanted to preach to before he even got an opportunity to do so.They,for the most part,rejected the message he brought and he eventually made his way to Rome.(Acts 22-27)
There was in fact a transition but not one of two different gospels,it was that of the understanding of the people of God as to the requirements of salvation.Many still were not understanding the changes Jesus' death brought as was predicted by the prophets and expounded by Paul in Hebrews 7-10.