That is what I am doing as well. I'm building my own kernel but not using secure boot.

Kernel upgrades are just copying .config from the old kernel to the new kernel.

sudo make olddefconfig && sudo make && sudo make modules_install && install_kernel

The contents of install_kernel

KERNEL=`readlink /usr/src/linux`

NUMBER=${KERNEL##linux-}

NUMBER=${NUMBER%%-gentoo}

sudo cp arch/x86_64/boot/bzImage /boot/EFI/Gentoo/vmlinuz-${NUMBER}-gentoo.efi

sudo efi bootmgr -c -L "Gentoo ${NUMBER}" -l "\\EFI\\Gentoo\\vmlinuz-${NUMBER}-gentoo.efi" -d /dev/nvme0n1