I wonder if something like '-gsplit-dwarf' would help - if the version of gcc they are using even supports it.

You can load additional symbols into gdb without restarting it by using `add-symbol-file`, which you can use to iterate on the symbol mapping.

I'm trying to start a decompilation project for a GameCube game any pointers from people here on how to go about it.

The other doesn't actually need to not modify the (elf) binary, meaning he could add symbols to it or whatever as extra metadata. What matters is that all of the on-cartridge assets don't have their addresses changed from the perspective of the system (emulator) and the section/segment headers.