Ok, but then why add the Covenant of Law at all? What purpose would it serve? Why not just fulfill the Covenant of Promise without any intermediary period of Mosaic Covenant (and with it, the seed as national Israel)? The law was added to illustrate, illuminate, incubate, impede but also aggravate the problem of transgression.
The only way to keep your Greek and Hebrew is to read Greek and Hebrew. I think you probably already knew that was the answer. You just didn’t want to admit it. But the languages are just like everything else: if you don’t use it, you’ll loose it. So what we really need is not a trick or a gimmick, but a reading plan. In the rest of this post, I will offer two.
The μυστήριον for Paul is less the “secret” of Christ’s Messianic identity and more like the “surprise” that the Gospel, as it is fulfilled in the resurrection and Pentecost, goes out beyond the Jews; it goes directly to the Gentiles too, and to the ends of the earth.
The question is really a matter of “how many exegetical decisions am I going to answer in my translation.” The more you leave ambiguous, the more burden you put on the reader. The more exegetical questions you answer, the less burden you put on the reader.
The point: when translating from the Greek, these subtleties won’t always show up in translation. That’s why it probably feels “low impact.” But such questions are worth thinking about because, though subtle, the rhetorical and semantic functions are different in many contexts.