Oh my god I like this theory! thank you for sharing your thoughts:)
I like this theory too. I guess it sounds firmer in the “There is no more Eurydice” so they switched to “No more Eurydice” for poetic appeal? Actually prior to chapter 205, my interpretation was it’s link to honesty to yourself; accept all parts of your past as inseparable pieces of your growth and move on without any regrets. Because in several Q&A, BDK always called Heejae is a ball of regrets, there’s even ask about what his life would be without Garam as well. So I kinda took that and ran with it haha. You reach part of the journey where both have to look at both good bits and flaws of the people you love the most and change for the better (no regrets).
After reading chapter 205 (thank you Sage!), I couldn't help but to chat with my sister about it. During our conversation, I raised that Kang Seomi had compared Heejae/Garam to Orpheus/Eurydice, but the reverse would also be true. My sister (who hasn't read the story) commented that perhaps they're both Orpheuses. Her comment got me thinking that maybe there is indeed "no Eurydice" as one of the potential translations of the title.
Both of them being Orpheus also lines up nicely with how Heejae and Garam charm others and even each other with their music. The current arc is not just Heejae's confrontation with his nature as a knife. Garam too has to confront his nature of "accepting" a knife. It's reached a climax where the two must decide how to deal with the difference in their natures (the pain that has quietly persisted as Garam had put it earlier on in the story).