as with your other questions and doubts, you always hit an important problem which allows me to develop things a bit further.
With regard to your observation that Mark is allegorical, and even more so is Matthew (although I would need to understand which parts you find allegorical, as there are certainly sections which are and others which are less), here is how I see it:
As Luke is the closest copy of Marcion’s Gospel, and Marcion’s Gospel is biographical in its basic structure (although it omits the birth and youth of its protagonist) – very similar to the geo- and historiographical structure of Marcion’s collection of Paul’s letters –, it is no surprise that Luke like Marcion’s Gospel is the one that sounds most biographical.
Yet, despite the straight copying of Marcion’s Gospel by Luke, Luke has altered many features of Marcion’s Gospel – by introducing a birth and youth story of Jesus, emphasising him as Lord, making the many links to his Jewish lineage and Davidian heritage and more. And yet, you are right, the biographical character it preserved, and even tried to strengthen through those additions. As Marcion’s biographical nature of his Gospel was antithetical, meaning that through biography and history, Marcion wanted to point out the non receptive nature of history and the incomprehensiveness of the Jewish people for the transcendent and unknown God and his Messiah, Luke counters this programme by his emphasis on history.
Mark, in contrast, deviates more in wording from Marcion’s Gospel, yet, he chooses a different approach to counter Marcion’s Gospel by, like Luke, adopting certain features, others than Luke. For Mark, the Gospel of Marcion disentangled Jesus from the Prophets, hence, Mark starts with making this link. He had less issue with Marcion’s criticism of history, on the contrary, Mark even emphasises the hidden and mysterious character of Jesus – therefore, he even pushes Marcion’s message more into this direction, something you call allegorical.
Matthew in his turn, picks up Marcion’s Gospel (presumably before Luke and after Mark) and is the one who extends Marcion’s Gospel with the birth story, underlines the historicity, but not as in Marcion, to dispute history as such. Instead, he turns Marcion’s antithetical relation between Jesus and the Jews (especially the leading groups, people and institutions) into an anti-Jewish position.
Hence, if you adopt my new dating of Mcn being first (but note – I am giving up the idea of straight dependencies of the Gospels, as I see only Mcn’s draft being the first Gospel, while his published version with the Antitheses has clearly known and read the canonical Gospels), I would rather think that we don’t see a straight move, but that a history critical historical biography (Mcn) created different responses, more allegorical ones (to save the mysterious – Mark, to save Jesus as heir of Israel – Matthew), and a more historical one (Luke with added Acts to also accomodate and position Marcion’s collection of Paul’s letters).