I wouldn't choose a name ending with an -a, as it is much the same sound as the -er in the surname. Something with the rhythm of Elizabeth would sound good. (Penelope, Felicity, Evangeline - I'm sure people can come up with plenty more)
I tend to agree with others about the a thing, but it's not like it is awful.
I like Alexa though (prefer Alexia or Alexi tbh) Alannah is nice though and there won't be many and I think the h (though silent) does make a different.
Charlotte, Florence, Catherine,Elisabeth and names like that would prob go well but may to slightly too traditional for you based on your names.
I think names -s or and e sound would probably go well. But I can't really think of any really good one that seem to be in the same ilk as your other names. How about. Eleri,Aoife,Bridie,Mhairi,Ffion,Ellie,Ellen,Grace,Molly,Millie,Rae,Alice
It doesn't bother me too much about the "a" ending seeing as my own name ends in a! I do like Charlotte and Scarlett but DH rhymes them with "the harlot" so that kinda rules them out! Thanks for the suggestions
My DC surname also ends in "-er" and we avoided any a, ah, er ending first names.
Leila Trayer: cannot say this Ella Trayer: Eritrea, definitely. Alannah Trayer: sounds a bit like a disease (sorry) Alexa Trayer: the best of these 4 but still I'd rethink the a ending names unless you can find a 3 or 4 syllable one. For example, with Victoria Trayer there is less blending from first to second name.