I use a simple script to update my kernel...

<code bash>
#!/bin/bash                                                                                                                                                                                                                                                                                                                     
cp /usr/src/linux/.config /usr/src/linux-5.0.0-gentoo/.config
rm /usr/src/linux
ln -s /usr/src/linux-5.0.0-gentoo /usr/src/linux
cd /usr/src/linux
make oldconfig
make -j9
make -j9 modules_install
cp /usr/src/linux/arch/x86/boot/bzImage /boot/vmlinuz-5.0.0
emerge @x11-module-rebuild
emerge @module-rebuild
genkernel --mdadm --lvm --busybox --disklabel initramfs
mv /boot/initramfs-genkernel-x86_64-5.0.0-gentoo /boot/initramfs-5.0.0.img
grub-mkconfig -o /boot/grub/grub.cfg
grub-install --boot-directory=/boot /dev/nvme0n1
cp .config /boot/config/5.0.0.config
bzip2 /boot/config/5.0.0.config
make clean
#eclean-kernel                                                                                                                                                                                                                                                                                                                  
#make mrproper                                                                                                                                                                                                                                                                                                                  
</code>

When a new kernel has been installed I use [sed](https://en.wikipedia.org/wiki/Sed) to replace the version...

<code bash>
sed 's/0\.0/0\.9/g' ~/bin/kernel-update
</code>

Usually works fine, check grub install output before rebooting.