something in this thread… to me it seems you are way over your head. That maybe ok, you can learn, you can break things. I for example don’t even use btrfs, but use grub, and live happily in my working linux distro…
just seems to me that changing your system to systemd created way more questions and problems than staying on grub. 
Edit: this concerns snapshots, but who knows what other complications come with btrfs…