you can definitely use c++ libraries compiled with clang++ when your code is compiled with g++ and vice versa. It only gets funky when one uses libc++ and one libstdc++
Ive never found linking to take more than 5 seconds with the standard linker which, but lld exists and its super fast in my experience. But i guess its only needed whon working on huge c++ like firefox or sth. like that.
prepare an encrypted partition on the new drive, then mount both the new and the old encrypted partition and rsync/cp -R everything over?
i guess you need to take care of the boot loader and maybe /etc/fstab separately
alternativly, clonezilla should be able to back up your partition if you cryptsetup open it but I've never used clonezilla so I don't know for sure