First, the idea that the Evangelist reasoned from the LXX of Isaiah 7.14 to the concept of the virgin conception seems odd. In Hebrew that text signifies nothing regarding a future Messiah conceived of a virgin. While the LXX wording fits better (ἡ παρθένος) the context still seems to demand much reworking by Matthew. In other words, I don’t find Isaiah 7.14 an apparent source for a virgin conception narrative. Rather, I think Matthew had the concept and he scanned through his knowledge of Torah for any evidence that Scripture foretold such an event. In other words, Isaiah 7.14 seems like it must be reworked to serve as a proof-text for a virgin conception more than it seems like an obvious prophecy that the Messiah would be born of a virgin.
Second, pagan deities being born of a virgin assumes a higher Christology than I find evident in Matthew’s Gospel. Matthew is more concerned with presenting Jesus as the Messiah (though we may say in proto-orthodox language as a Messiah within whom YHWH dwelt in a unique way) than as a “deity” in the pagan sense of the concept. So sure, there are examples of pagan gods being born of virgins. I’m not convinced this motivated the Jewish Evangelist.
Of course, this does not prove a virgin conception, but I don’t think these proposals are as sound as some argue as evidence against it.