[kps]

> I have also been told by more than one person that does professional compiler development that GCC's codebase is very difficult to work with.

Add another. (Now-former professional compiler developer.)

> At least some commentary I've read suggests that this was a deliberate choice on the part of the GNU project.

This is the ‘RMS loophole’ in the GPL. In theory you can do what you want with the source; in practice you need help from the insiders.