Sure, you can gloss 'panta ta ethnE' as 'all the tribes' , or go full out and give it its Attic sense of 'all the companies of men'.
The problem for you position is that, throughout the LXX, 'ta ethnE' used as a translation of goyim, and usually Englished as 'the nations' or 'the gentiles'.
Get out your Greek Old Testament and read Psalm 1, and there it's sometimes Englished at 'the heathen'. Never have I seen Psalm 1 translated "Why do the tribes rage. . ."
The Great Commission, read using the Greek the way it is used throughout the Scriptures, is explicitly a command to convert and baptize the gentiles.
And Semitic or not, the Canaanites were the gentiles ne plus ultra!
And you are wrong: the reason the detail of the Ethiopian court offical being a eunuch was important: eunuchs could not convert to Judaism.
The problem for you position is that, throughout the LXX, 'ta ethnE' used as a translation of goyim, and usually Englished as 'the nations' or 'the gentiles'.
The problem is that the same term (goyim) is also used for the non-believing Jews/Semites in the Old Testament. All pagans were goyim.
+Paul changed the term to mean non-Jewish Christians.
But the real problem is that Christ neither taught, sought nor brought goyim to His ministry, nor did He direct the original Apostles to preach to and convert the goyim. Never. His ministry was for the Jews and about the Jews and by the Jews; now you tell me it suddenly changed in Mat 28:19 because of the Greek word (the Greek word for God's justice is also a poor translation of Hebrew, so this is not the only transaltion fallacy).
But one thing is certain: He not once suggested that Judaism is to 'morph' into something esle, to dispense with dietary laws, with circumcision, etc. Not once.
And you are wrong: the reason the detail of the Ethiopian court offical being a eunuch was important: eunuchs could not convert to Judaism
Apprently, to Christian Judaism yes. This was part of the reason the Jewish Christians were seen as heretics in Israel.