We used
Mary as a mn for our first daughter after her late grandmother who had been
Bertha Maria and was Afrikaans. We preferred the sound and length of
Mary in context (
Caroline Beatrice Mary), while not disliking
Maria at all. With shorter names around it,
Maria would be fine.
I wouldn't like to use
Marian. I'd expect it to sound like Mary+un, but I mostly hear it like
Marion, with a short -a-, which I don't like.
Marian is also too adjectival and therefore too Catholic!
I love
Marianne though - happy memories of Sense and Sensibility - and would use it cheerfully as a fn.