7 string player here. Tune the seventh to a low B of occasionally Bb or A, depending on the context. A lot of jazz players tune 7 to A as this makes playing chords very easy. You can use shapes which usually have the root on the fifth string and just put the root down lower instead. I don’t like this, I prefer the new chord shapes that appear with 7=B, however for ease of learning you can’t beat the 7= A strategy