Alexandra- Love it. Prefer it unshortened but prob unrealistic.
Anastasia - Bit fussy for me but quite nice. I knew one who went by Stasi which could be an alternative to Ana.
Violet - it's ok but don't especially warm to it.
Emma- nice enough but yes, I do think of it as quite plain, I think.
Alyssa or Alicia (a-li-see-ah) - both nice. I think I prefer 3 syllables. In my head Alicia is Aleesha but I'm not sure why.
Theodora/Theo - not keen on this at all. Very 1940s boyish.
Helena/Lenny/Laney - It's ok. I know a Helena (Irish) who pronounced it Hel-een-a rather than Hel-ey-na or Hel-en-a. Think I prefer Hel-en-a
Robyn - love it. Very pretty and underused.
Juliet - love this too. Surprised this isn't more popular.
I'd shortlist to Alexandra, Robyn and Juliet from your list.
Other possibles:
Abigail, Bronte, Cassandra, Daphne, Eleanor, Francesca, Giselle, Henrietta, Imogen, Joanna, Lydia, Miranda, Natasha, Octavia, Philippa, Rhiannon, Sophia, Tamara, Vanessa, Xanthe