Final update:
So on a hunch (and after doing further investigation online which led me to an obscure post on the Ubuntu forums that involved a similar situation), I decided to unplug the USB cable from the internal USB header on my motherboard. I booted up the computer, and lo and behold, the Bluetooth toggle was GONE. So I will consider this matter "closed," and we can move on, but I will say here (since I've already tagged Artyom and all), I hope improvements will be made to Zorin OS that enhances the user experience (power management, Bluetooth settings, and the GNOME system menu with quick settings and toggles) and makes these kind of things more accessible. I'm not sure why the "systemctl" command didn't work here. Should've completely disabled Bluetooth and have had the same effect that unplugging the USB cable did. Interesting. Anyway, the problem is solved (to me, anyway), and we can all move on to face another day in this earthly realm. Cheers.