I kind of love the name [name_f]Della[/name_f], but [name_f]Ella[/name_f], [name_f]Elise[/name_f], [name_f]Eleanor[/name_f], etc are insanely popular where I live. Basically anything that can be shortened to [name_f]Ellie[/name_f]. [name_f]Do[/name_f] you think [name_f]Della[/name_f] is different enough or probably going to be just added to the pile of Ellies?
To my ear, [name_f]Della[/name_f] has a distinctive sound. I think having “D” as a first initial would set her apart.
I think [name_f]Della[/name_f] will stand out just fine. It’s short enough not to need a nickname so both the first letter and the -a ending will set it apart from Ellies.
Another idea and a name I slightly prefer is [name_f]Delta[/name_f].
I love [name_f]Della[/name_f]! I intend on using it as nickname for [name_f]Delphine[/name_f].
It’s a very pretty name
[name_f]Della[/name_f] and [name_f]Delia[/name_f] are lovely names and much more interesting than the others you mentioned. I’ve never taught a [name_f]Della[/name_f] or a [name_f]Delia[/name_f] in my 30 years as a teacher.
My husband is also a teacher…man, that makes it hard to name babies with him! Difficult students have ruined a lot of names for him! 
I think it’s different enough and will actually set her apart from all the [name_f]Ella[/name_f]/[name_f]Ellie[/name_f] names.