I am a little sad that there is no route for Argos, he turns out to be such a good boy that I can't help but be drawn to him. I 'd even settle for a short route like " I know what you feel and i feel the same too, but this... us... I'm afraid it was never meant to be, but for tonight... if only for this one night... let us throw caution to the wind and enjoy this small reprieve from the meddling of the divines." before the game shifts back into the main routes, occasionally referencing back to what could have been, perhaps sidelong glances or a lingering touch, a quick peck on the cheek before parting ways once more, both in a sad understanding of what they cannot have. This would also give Argos a new level of appreciation for helping MC to love Asterion, only wanting to see MC happy above all else.
A poly relationship likely isn't in the cards, there's only been one thing I could point to that compared to the boundless other examples against it, but that's the only way I see a romance working. Not just because Asterion needs trust, so a scene like what you're describing would be devastating, but also because Argos genuinely wants those two to be together. I do see something between the three (depending on some hyper-specific choices to see what I mean), which is close to what I'd consider an ideal relationship where it's 3 people with equal feelings, but at the end of the day this is a story about breaking cycles and I think it runs too close to repeating Laomedon's split interests if any other love interests, poly or otherwise, are added.
(I would obviously be happy to be wrong, though! I just think it's harder for a story as seeped in symbolism as this to pull off in a way where the significance isn't muddled.)