I pronounce
Johanna like jo-HAH-nah (with -hanna rhyming with sauna, more or less) because that's the way the
Johanna I work with pronounces it. Jo+Hannah would be my second guess. I wouldn't think to pronounce it like
Joanna unless I was told to.
I like
Joanna and
Johanna. Johannah looks like it has too many letters, and
Joana doesn't look like it has enough.