systemd-boot writes itself to $ESP/EFI/BOOT/BOOTX64.EFI, which grub doesn’t overwrite unless you have efiInstallAsRemovable = true;, which is mutually exclusive with canTouchEfiVariables = true;. Given that canTouchEfiVariables = true; is preferable, IMO, just delete $ESP/EFI/BOOT/BOOTX64.EFI.
Also, don’t set efiSysMountPoint = "/boot/efi"; unless you have actually moved your ESP to /boot/efi. And even then… why? I would recommend against it. Just having /boot and the ESP be one and the same is preferable IMO