If you are such a person and wish to use a guitar, do yourself a favour and find the intermediate chords or runs. Your voice requires it. If you can't, try learning more chords anyway. Your singing will improve, I promise."
your overall performance will improve, agreed, if one wants to improve singing, there are several other things ,posture, breathing, practice, listening critically to ones own performance, and thought about presentation, voice intonation can be improved by singing chromatic scales or any scales or intervals with an instrument.