Where I live,
Johan = you(schwa)-HUN because long vowels tend to diphthongise with a neutral vowel. Which means that
Johanna = you(schwa)-HUN-ah. Of course, Afrikaans speakers in the States are thin on the ground.
If you want the jo-HANN-a version, what about upper-casing the H - JoHannah? I think that should make your intentions clear!
Johanna is pretty old and tired; Johannah never happens;
Joana would be Portuguese if anyone used it (in my experience they don't, not here anyway) and
Joanna is very beautiful indeed and well worth using.