If you build a cross toolchain then make sure you automate the build
Man, I have 15 years of experience with gentoo.
Everything (including experimental stuff) I do for whatever architecture I have to support { MIPS32BE, HPPAv2, POWERPC, X86} is done by Overlay recipes, while the whole rootfs is under Catalyst, and Catalyst compiles every new rootfs as chroot.
The main rootfs is safe and never update until things have passed the stable level (whose severity level is more fussy than on Debian-stable, I mean I am fussier than those guys).
Things are under control, but GNU sucks a lot, and their FSK software is always broken, indeed even guys behind Ubuntu, Centos, etc, have the same problems I have every FSK time we need to update something.
Now the whole GlibC is ... fragile
