Guys - you made my day. Thank you so much.
Together with your input and this Gist I found, I was able to restore GRUB 
su
mount -o subvol=root /dev/nvme0n1p7 /mnt/
mount /dev/nvme0n1p6 /mnt/boot
mount /dev/nvme0n1p1 /mnt/boot/efi
mount -o bind /dev /mnt/dev
mount -o bind /sys /mnt/sys
mount -o bind /proc /mnt/proc
mount -o bind /run /mnt/run
mount -o bind /sys/firmware/efi/efivars /mnt/sys/firmware/efi/efivars
chroot /mnt
mount -a
dnf reinstall shim-* grub2-*
grub2-mkconfig -o /boot/grub2/grub.cfg
efibootmgr -c -d /dev/nvme0n1p1 -p 1 -L Fedora -l '\EFI\fedora\shimx64.efi.efi'
After this I was able to boot Fedora via UEFI Boot Menu. Then I executed this and after restart, GRUB is shown again:
dnf reinstall shim-* grub2-*
grub2-mkconfig -o /boot/grub2/grub.cfg
Crazy challenge but again learned a lot about Linux 