That’s interesting you mention about who Romans was written to because lately, as of yesterday or the day before I think, I suddenly noticed that the audience in Romans did not seem to be Gentiles, but rather Jews. Though I do think there were probably some Gentiles, too, but who knows for sure.
I firmly believe that who a letter is addressed to changes the delivery and reception of it. Such as Hebrews for example.
Paul’s letter to the Romans seems to have broken with his mission to the gentiles, so he didn’t have apostolic authority in Rome, nor did he like building a foundation where others had already built. Paul was going to go to Spain, through Rome, in order to church plant.
It's Definitely an interesting study of Paul's epistles, to see which letters were directed at the Jewish believer and those at the Gentile believer. Ephesians is definitely directed at the Gentiles, and because they didn't have the confusion the church in Rome had, they became one of the Lords seven founding churches.
I often wonder, if any of the seven were Jewish.