I am trying to define a new shortcut for "Select the previous input source", since the default shortcut (Control + Space) is conflicting with another app I need to use. Is there any table with a list of unused shortcuts in Mojave that could be good candidates for custom shortcuts? I am trying to find a balance between good usability (not having to stretch the hand in awkward positions to type the shortcut) and avoiding conflicts with already existing shortcuts.
Is there any table with a list of unused shortcuts in Mojave that could be good candidates for custom shortcuts?
There is not. There are a few utilities out there which will show you which key combinations are in use¹ but none (that I am aware of) show you available keyboard shortcuts.
¹ I believe KeyCue is the most well-known and best supported.
One common method among Mac power users is the use of a Hyper Key which is (generally speaking) where you remap holding down the Caps Lock key to equal holding down ⌘ ⇧ ⌥ ⌃ because few apps (and few macOS default shortcuts) use all 4 modifiers (mostly because it is awkward to try to press them all at once, which is why remapping the Caps Lock key is recommended).
Personally, I use a Hyper Key with Keyboard Maestro because, unlike shortcuts defined in System Preferences, Keyboard Maestro's shortcuts are easy to backup/restore, and easy to sync across multiple Macs.