Circumcision

Paul himself had Timothy circumcised
That wasn't Paul deciding circumcision had become necessary for Christians. Timothy was circumcised for missionary/customary reasons, so his Jewish background wouldn't unnecessarily prevent Jews from listening to the Gospel.

Acts 16:3

“Paul wanted Timothy to accompany him; and he took him and circumcised him because of the Jews who were in those places, for they all knew that his father was a Greek.”

Paul refused to allow Titus to be circumcised when false brothers were trying to make it a requirement:

Galatians 2:3–5

3 But even Titus, who was with me, was not forced to be circumcised, though he was a Greek. 4 Yet because of false brothers secretly brought in—who slipped in to spy out our freedom that we have in Christ Jesus, so that they might bring us into slavery— 5 to them we did not yield in submission even for a moment, so that the truth of the gospel might be preserved for you.

Timothy: circumcision was voluntary and useful for avoiding an unnecessary cultural obstacle to evangelization → Paul permitted because Timothy's Mother was a Jew and father was a Greek so the Jews wouldn't accept or listen to them if he wasn't circumcised.

Titus: circumcision was being demanded as a religious requirement → Paul refused.