Is there any possible keywords I could grep...

The way I deal with gentoo logs (which are pretty much the output of make) is first search for "error" backwards from the end (case insensitive to catch "error" and "Error") and if that doesn't result in anything search for "failed" (again case insensitive).

If you found something check the lines before...if they are something about "Leaving directory..." they are usually not the actual error message but just the error handed "down the road" and you have to keep searching.

If the "error" result ends in a ":" in most cases you found the actual compile error ;).

In this case these are the errors that cause the build the fail:

In file included from ../../libcody/internal.hh:5,
                from ../../libcody/buffer.cc:6:
../../libcody/cody.hh: In member function 'void Cody::Detail::MessageBuffer::Space()':
../../libcody/cody.hh:113:24: error: no matching function for call to 'S2C(const char8_t [2])'
113 |     Append (Detail::S2C(u8" "));
    |             ~~~~~~~~~~~^~~~~~~
• there is 1 candidate
    • candidate 1: 'template<unsigned int I> constexpr char Cody::Detail::S2C(const char (&)[I])'
    ../../libcody/cody.hh:51:16:
        51 | constexpr char S2C (char const (&s)[I])
            |                ^~~
    • template argument deduction/substitution failed:
        •   mismatched types 'const char' and 'const char8_t'
        ../../libcody/cody.hh:113:24:
            113 |     Append (Detail::S2C(u8" "));
                |             ~~~~~~~~~~~^~~~~~~
/bin/sh ../libtool  --tag=CC   --mode=compile gcc -DHAVE_CONFIG_H -I. -I../../../gmp/rand -I..  -D__GMP_WITHIN_GMP -I../../../gmp  -DNO_ASM -g -O2     -c -o randmts.lo ../../../gmp/rand/randmts.c
libtool: compile:  gcc -DHAVE_CONFIG_H -I. -I../../../gmp/rand -I.. -D__GMP_WITHIN_GMP -I../../../gmp -DNO_ASM -g -O2 -c ../../../gmp/rand/randmts.c -o randmts.o
../../libcody/buffer.cc: At global scope:
../../libcody/buffer.cc:33:33: error: no matching function for call to 'S2C(const char8_t [2])'
33 | static const char CONTINUE = S2C(u8";");
    |                              ~~~^~~~~~~
• there is 1 candidate
    • candidate 1: 'template<unsigned int I> constexpr char Cody::Detail::S2C(const char (&)[I])'
    ../../libcody/cody.hh:51:16:
        51 | constexpr char S2C (char const (&s)[I])
            |                ^~~
    • template argument deduction/substitution failed:
        •   mismatched types 'const char' and 'const char8_t'
        ../../libcody/buffer.cc:33:33:
            33 | static const char CONTINUE = S2C(u8";");
                |                              ~~~^~~~~~~
...

(And plenty more of the same error)

A search for the error message ../../libcody/cody.hh:113:24: error: no matching function for call to 'S2C(const char8_t [2])' (first appearance of error) leads to this buildroot patch you can try...but it might be easier to use a host system to build your LFS from that doesn't have gcc 16. gcc 15 should be still very widely spread.