How difficult do you estimate it would be to port this to linux?

I'm not asking you to do this, of course. But if you were willing to let someone else fork this, it would be a huge improvement over sxhkd on linux.

How separated is the code for handling keypresses? (Ie the OS-specific stuff)?

Start by tracing the keypress-handling path in the repository and identifying which parts are tied to macOS. Compare that boundary with the Linux requirements described in the issue. Done would mean a documented feasibility assessment and a clear scope for a Linux port, rather than an implementation patch.

Written by the indexing model from the issue text.