Heejae is famous for his beethoven and rachmaninoff performances because they tend to be intense and flashy, just like his outward personality. His chopin on the other hand is more understated and "boring" for some people, but garam prefers it because he understands that heejae's chopin reflects his loneliness and vulnerability.
Chopin was the first piece heejae played when they first came to his private residence in NY, depicting garam peeking into who heejae truly was. Watching heejae plays chopin on the stage for the first time was when garam completely fell for him. And listening to garam's drunk rant about the same chopin performance was also the first time heejae felt connection and kissed him. So yeah, chopin represents heejae's more delicate and sincere inner self, and his love for garam.

Heejae is constantly reminded of the manager because what happened between them was one of the biggest regrets of heejae's life. So whenever heejae is feeling vulnerable with garam, he'll remember the manager like a warning sign. It's like a cycle > he feels vulnerable, be it in happiness or anger > he lashes out at garam > he's reminded of the manager who left him > he's scared that garam will leave him because of that.
So it's not that garam is a replacement, it's just that heejae didn't have a lot of people he cares about in his life, and he's scared that he'll repeat the mistakes he did with garam (whom he loves, immensely)
Also, i love that heejae always plays chopin whenever he feels vulnerable/ exposed gggghhh so beautiful.