I play mostly bass dont worry. I think the explanation is odd but easy. I tried singing guitar solo (little wing etc). Tried to expand range, and when it was above, I started to try to sing other instruments or backing vocals. Started with unison and fifth, then 9th etc etc. I also listened to contemporary gospel and acapella (take 6, brothers voices only, sometime they imitate instruments too, Google take 6 - jazz vive).

Also jazzfusion guitar chord progression got me to focus on funny progression. John scofield, tribal tech, r. Gatto, return to forever...) All reach your ears.