I just want to use this shortcut for one other application.
In Mojave, selecting an Input Source as "Unicode Hex Input" in [ System Preferences > Keyboard > Input Sources ] does the trick.
Note that when you click the + to add this Input Source it appears way at the bottom under "Others".
Make sure to enable "Show input menu in menu bar" and select the new input method from the menu bar since it doesn't seem you can make this the default from the Control Panel.
To type a unicode character in Unicode "Input Source" mode, hold Option ⌥ and type the 4-digit unicode:
- Option ⌥ + 2120 = ℠
- Option ⌥ + 2325 = ⌥
Unfortunately, this is not a great solution for anyone needing a layout other than the basic English one. Some might find it suitable to just use the menu bar to switch to this input source when needed (ie. coding) and back to the native one otherwise.