I wish it was so easy as on MacOS or Windows - where you can essentially turn it on with one button. I don't think enabling encryption on an existing installation of Linux can be done without reinstalling or a lot of work.

Ackchually, enabling full disk encryption via LUKS is rather easy on Manjaro. It's literally a button that states "use full disk encryption" as part of the setup wizard. You then enter a password and that's it.

The tricky bit is if GRUB breaks (hint: GRUB looks for every opportunity to break. If it can break, it will) and you have to chroot into an encrypted LUKS partition. That's where your average user will be SOL. Not sure how the recovery process would work for Bitlocker or FileVault.

Leace

For modern setups that support UEFI, GRUB is unnecessary as you can boot Linux kernel directly as EFI executable.

If one cares about SecureBoot then this package automates everything: https://github.com/andreyv/sbupdate (it's setup only once and works later without supervision).