A few assorted thoughts:
I like Julianna or Sophia because I naturally want a non-A name (and I find Trinity too different to Alexandra and may be less culturally mobile).
Having said that, Annabell (haven’t seen this version often!) suits Alexandra, in some ways more than Sophia (and I would also say that Annabell is a nod to Isabel so you could then cross that off the need to honour list). But just prefer a non-A name if possible.
Seeing as you like smoosh names, how about, non-A anna names:
Lillian, Lilianna or Lily-Anne (Lily + Anne nn Lilly)
Rosanna (nn Rosie or Annie)
Susannah as a first (nn Lily - or Suki - which also honours Suzanne)
My other brief thought is that I’m not sure about the suggestion above of Suzanne as a middle with Annabell as a first or any “Anne” name - eg Julianna - unless you like the call back (ie two Anne names in one name)? In other words, Juliana honours Suzanne and Ann (and Lily and at a push Ellen).
Similarly I wouldn’t use Mabel as a second if the first name is Annabell (too many Bells). How about just one Bell name (eg Annabell and that honours both Isabel and Mabel?).
Also, I just thought of this, if Lexi’s middles are Lily Ann, then maybe having both Lily and Anne/Anna in the next child’s name might be a bit derivational/ too much for a second child? On the one hand I get it’s lovely to have the connection but as a middle child, I wouldn’t want much of my name stemming from my older sister’s - unless it was the exact name as a middle (ie one of the middles was Lily for eg so we both had Lily) and no more. So I wouldn’t use Anne and Lily in the name esp the first name unless you would like the repeat and are banking on a perfect sibling relationship. If you pick Julianna, I wouldn’t use Lily as a middle name as Lexi’s Ann already in there - and even Lily really with the Li in the name.
With Julianna, other names that suit Alexandra: Charlotte (Lottie); Felicity; Louisa; Elisabeth (nn Lil/Lily); Josephine (Josie, Posey, Phina) - and I like Sybella too (nod to Isabel and Mabel, nn Bella/Bell/Sissy).
Middle options:
Laura Mabel
Susan Mabel
Laurel Susannah (Laurel being Laura + Mabel and meaning of Laura)
Susannah Mabel
Mabel Suzanne
Helen Maisie
Ellen Maisie
Helen Laura
Lily May
Belle Suzanne
Belle Susannah
Eleanor Mabel
Annabell Elise
Lily Mabel
(Mix and match so avoid double Anne, double Bell etc).
Hmmm… hope these random thoughts make some sense. Julianna the standout I’d say!