To learn how to sing without stress is knowing the right key to sing with, a key that will not strain your voice, a key that you can sing on your high and low register. It's not a must to sing a song on the same key when ministering, I don't know where choir masters and music directors got that from. This has lead to so many uncomfortable singing on stage with so much off keys, flat notes and shouting because the choir masters made the singers minister the song in a key they are not comfortable with.
In a case that a singer is working on his/her pitch, you are permitted to select songs higher than your normal pitch to practice with, so that you can improve on your pitch and range. In these process of training you will surely feel uncomfortable cause you are not use to singing like that, your voice will crack, but with time you will start becoming comfortable with that pitch and your pitch will increase. But please this is only for personal rehearsals, don't go and minister with a key that is not comfortable for you please it will lead to off keys and vocal cracks.
Learning other techniques like riffs and runs, vocal placement, vibrato, pitch, falsetto etc will help you gain excellence and have more freedom in singing effortlessly. So don't be mediocre and settle for less cause there is more you can do with your voice, so I challenge you today to get a vocal coach that will guide you towards being the best version of yourself.