I do not have broad knowledge of early church Father views of Genesis but I know enough to say that some believed in a Trinitarian view, some did not. Some believed in literal 24 hour days with the earth being created only thousands of years before Christ, while others did not. Most mixed Greek philosophy with their views.
Besides the rejction of mixing Greek philosophy with his interpretation, Luther must have felt extra 'alone' in that he did not think to highly about many of the church Fathers and the two he admired the most, Hilary and Augustine, they did not take it literally. This meant even his favorites must be left behind in his huge endeavor (5 Volumes of interpreting Genesis in a fresh new light).
This is what Luther say's himself in his introduction to Genesis which presents his attitude about it all. First, your right he does say he is the first to clear things up on a significant scale:
Until now there has not been anyone in the church either who has explained everything in the chapter with adequate skill. The commentators, with their sundry, different, and countless questions, have so confused everything in the chapter as to make it clear enough that God has reserved His exalted wisdom and the correct understanding of this chapter for Himself alone, although He has left with us this general knowledge that the world had a beginning and that it was created by God out of nothing. (Luther's Works Vol 1, Introduction)
He pays respect to Hilary and Augustine but then proceeds to absolutely reject their symbolic view:
Hilary and Augustine, almost the two greatest lights of the church, hold that the world was created instantaneously and all at the same time, not successively in the course of six days. Moreover, Augustine resorts to extraordinary trifling in his treatment of the six days, which he makes out to be mystical days of knowledge among the angels, not natural ones. (Luther's Works Vol 1, Introduction)
Luther proceeds to reject the mixture of Greek philosophy with Genesis interpretations which it appears many theologians did and he names Nicholas of Lyra as particularly guilty on this matter:
As to Lyra’s belief that a knowledge of the philosophers’ opinion concerning matter is essential because on it depends the understanding of the six days of activity—I am not sure that Lyra understood what it was that Aristotle called “matter.” Unlike Ovid, Aristotle does not designate the shapeless and crude chaos as matter.12 Therefore, disregarding these needless opinions, let us turn to Moses as the better teacher. We can follow him with greater safety than the philosophers, who, without benefit of the Word, debate about unknown matters. (Luther's Works Vol 1, Introduction)
I think you can gather from these points that Luther was starting something new when he adopted a simple interpretation without a) admixture of Greek philosophy, without b) using symbolism and without c) ignoring the Trinity. Although there were others before him who had a similar view none appear to have taken all three approaches together, at least none whose works were extant and available to theologians at the time.