Oh this is where the OP ended up. I think it’s a fruitless effort to do a full translation, but I can do some impressions at least.
The song seems to be based on the narrative we’re seeing in Shiroha’s story extracts, not very surprising. From the extracts and the previous teaser, the motifs and phrases surrounding Shiroha and Hairi are: Waiting, Something Lost/Forgotten, Keep Walking, Looking Back (Pretty Key/Kanon-esque). The song also has a lot of this. Temporally, the song is a bit weird: the first half seems to talk about the future while the second half is reminiscing.
Some of the first few lines are like: “I thought I knew what to say,” “I turned my eyes away,” “I looked down seeking courage.” Quite a lot of awkwardness, then it talks of the end of summer building up to a “On that day, I want to wave my hand with a smile.” Okay, seems pretty straight forward about Hairi going back home. After that I’m not sure what happens.
The second half has stuff about needing to keep walking, and searching for a warm hand, but also not being able to reach some place, and then the final line like about looking back on summer memories. It becomes less straight forward. We still have the same motifs, but it’s not as much of a narrative, at least with the information we currently hold. This is completely standard fair though because we don’t have all the cards yet.
Edit: The title is still all Greek to me.