Relative to the details seen in the accounts of the Jews, Samaritans and Gentiles, you said, "in these scenarios, we see something very specific happening, in particular, the Gospel going to entirely new people groups." One could come to the conclusion that what they experienced was for that sole purpose if not for the account in Acts 19.
The Acts 19 account reveals all individuals are required to submit to water baptism in Jesus name and also experience receiving the Holy Ghost. The account is explicit; going so far as to specifically state the actual number of individuals in verse 7. As such, the account had nothing to do with nationality. What occurred was 12 individuals experienced being reborn of water and Spirit.
Each and every detailed conversion account reveals exactly what Jesus meant when He said, "Except a man be born of water and Spirit he cannot SEE or ENTER into the kingdom of God." The truth is in plain sight. Look; Acts 2:38-41, 8:12-18, 9:17-18, 10:43-48, 19:1-6, 22:16.