I see this name & say Sanna like it rhymes with Anna (ann-uh). Not sure if that's the proper pronunciation.
I think it could work as a Suzy/Donna smush with legitimacy behind it.
Do you have Scandinavian heritage as well?
I think it goes fine with your sons name.
I think that where I live no one would give you a weird look for this name, but in more rural areas people might be like, "Savannah?" "Santa?" "Sandra?" so if you can handle that I say go for it, but that unfamiliar aspect might make it a tougher name to wear.
I'm torn by this name. Upon hearing it, I think of either the word 'sana' which is 'healthy' in Spanish. Or my mind wants to add onto it and say san-itation.
So yeah, not for me.
I think other good ideas to honor both of those names would be:
Dana or Danna (Day-na and Dan-uh)
Yes, stretches... but they have similar letters and sounds to Suzy and Donna without being as out there.