Nice of you to make up a reason for Paul's behavior. Paul and every other early Christian didn't have an "old" testament. Their bible said:
Lev 23:2 Speak unto the children of Israel, and say unto them, Concerning the feasts of the LORD, which ye shall proclaim to be holy convocations, even these are my feasts.
Lev 23:41 And ye shall keep it a feast unto the LORD seven days in the year. It shall be a statute forever in your generations: ye shall celebrate it in the seventh month.
Act 18:21 But bade them farewell, saying, I must by all means keep this feast that cometh in Jerusalem: but I will return again unto you, if God will. And he sailed from Ephesus.
Paul didn't tell his fellow Christians "I have to go evangelize in Jerusalem". He specifically said that he was going to "keep the feast".
1Co 5:8 Therefore let us keep the feast, not with old leaven, neither with the leaven of malice and wickedness; but with the unleavened bread of sincerity and truth.
He instructed Christians to "keep the feast".
Paul observed the holy days of Jesus Christ because Christ created HIS holy days. When Christ incarnated, he observed the holy days and Paul imitated his example:
1Co 11:1 Be ye followers of me, even as I also am of Christ.
1Co 11:2 Now I praise you, brethren, that ye remember me in all things, and keep the ordinances, as I delivered them to you.
Your explanation for Paul's behavior just doesn't match up with scripture.
You seem to have a very stunted view of Pauls ministry.
for he vigorously refuted the Jews publicly, showing from the Scriptures that Jesus is the Christ. (Acts 18:28)
And he went into the synagogue and spoke boldly for three months, reasoning and persuading concerning the things of the kingdom of God. But when some were hardened and did not believe, but spoke evil of the Way before the multitude, he departed from them and withdrew the disciples, reasoning daily in the school of Tyrannus. And this continued for two years, so that all who dwelt in Asia heard the word of the Lord Jesus, both Jews and Greeks. (Acts 19:8-10)
Im curious, where exactly is it recorded that Paul made it to Jerusalem to keep the feast. It appears he was very busy evangelizing his fellow Jews, staying two years in Asia. It not even clear that he made it in time for Pentecost (Acts 20:16).