Where to start?

I am an accomplished guitarist having played professionally with a few different bands along with limited singing. I can read and understand music and music theory. Can play vocal melodies on an instrument and copy or hit the same notes with my voice to some extent. But recognize that my vocal quality needs a lot of improvement. What specific lessons or lesson series do you recommmend for someone like me?

That's a fantastic background! You've got lots of tools already to make improvement with your voice. I recommend the level 1 beginner course for a broad foundation in singing technique, followed by level 2 for in-depth development of your vocal registers and more vocal control.