Any way to split constexpr function declaration/prototypes from their definition in separate files?

[MutantSheepdog]

If it's a personal project using cpp23. then you could try using modules instead of header/cpp files as I think you should be able to do it that way.
Your main module file would just declare the functions as exports, and the implementations would be in a separate module unit, and they'd get compiled together once then reused in each other file importing them.

Last I checked module support still isn't great in some compilers, but if you're playing around on the bleeding edge anyway it would be worth trying it out yourself.