So many of your picks would work imo!! I’ve given a bit of an explanation for the names that stood out the most for me (I feel like this is more helpful than me looking at your list and going “Not my style, not my style” or something like that), and why they work, and also some points you may want to consider (bc it’s still a long list, LOL).
[name_f]Abigail[/name_f] - your names give me a classic, yet early 00s feel, as most of the kids I know with your kids names were born pre-2005 (it’s also something I [name_u]LOVE[/name_u] about your set!). For me, [name_f]Abigail[/name_f] would fit this pattern nicely, and lends itself to the adorable [name_u]Abby[/name_u]/[name_u]Abbie[/name_u]/[name_u]Abi[/name_u]. My only reservation would be that a) you’d have both girls with A initials, and b) nicknames seem like they might be inevitable with [name_f]Abigail[/name_f]. I don’t know how you feel on either point, but they’re things to maybe consider.’
[name_f]Ava[/name_f] - Super cute, fits the on-trend in the 2000s vibe, BUT in terms of rhythm, it feels a bit too short at 3 letters, when the other kids are 5, 6 and 7. Also, another A name. I still adore [name_f]Ava[/name_f] though, you can’t really go wrong with it.
[name_u]Avery[/name_u] - I adore [name_u]Avery[/name_u], it’s a frontrunner for when SO and I have children, however with your list, it feels a bit too unisex, not that it’s too masculine or anything, but it does kind of go against the classic themes because it ranks relatively high for boys as well.
[name_f]Charlotte[/name_f] - Fits the classic theme of your other children’s names beautifully - [name_u]Bradley[/name_u], [name_f]Audrey[/name_f], [name_m]Ethan[/name_m] and [name_f]Charlotte[/name_f] feel perfect imo. I can’t think of any real negative points or things to consider with [name_f]Charlotte[/name_f]!
[name_f]Eleanor[/name_f] - Again, fits the classical theme, but with [name_f]Eleanor[/name_f], I can’t help but feel that it’s a bit stuffier compared with [name_u]Bradley[/name_u], [name_m]Ethan[/name_m] and [name_f]Audrey[/name_f]. Your other children’s names give me a light, playful vibe, where as [name_f]Eleanor[/name_f] feels more stiff and buttoned up.
[name_f]Hannah[/name_f] - I feel like [name_f]Hannah[/name_f] ticks all the boxes for me - it’s classic, it peaked in the 2000s, it has a different first initial from the other names - it’s a perfect choice.
[name_f]Katherine[/name_f] - I’d say this is super similar to [name_f]Eleanor[/name_f] for me, it’s very buttoned-up and rigid feeling imo, and while I like it and it does work, it doesn’t vibe the same way as your other kids names do. I’d save this and [name_f]Eleanor[/name_f] for a MN spot though.
[name_u]Madison[/name_u] - Despite being unisex, I think [name_u]Madison[/name_u] works really well! It’s unisex but doesn’t rank for boys in the top 1000, which makes it less unisex in feel, imo? It has the same popularity trend as [name_f]Hannah[/name_f] and [name_f]Abigail[/name_f], and it shares a different initial. I think it’d be a really sweet choice!
[name_f]Olivia[/name_f] - Again, I don’t think you can really go wrong with [name_f]Olivia[/name_f]! My only reservation is that it doesn’t seem to shine as brightly for me as the other picks do. It’s still a favourite of mine, but I feel like other names work just a little better?
[name_f]Savannah[/name_f] - [name_f]Savannah[/name_f] is another choice that honestly, just feels perfect! I can’t think of a single reason to veto [name_f]Savannah[/name_f]!
[name_f]Scarlett[/name_f] - [name_f]Scarlett[/name_f] is lovely, it’s a bit more modern than the others on your list, but it still works with your other kids names! It also shares a bit of an old-Hollywood feel with [name_f]Audrey[/name_f] (and [name_f]Olivia[/name_f] too, I suppose!), which is sweet. Not my top choice, but it works well!
[name_f]Sophia[/name_f] - [name_f]Sophia[/name_f] is cute, but feels a bit overdone imo (sort of like [name_f]Olivia[/name_f]), which is honestly the only reason I’d shy away from this one.
[name_u]Sydney[/name_u] - [name_u]Sydney[/name_u] is adorable and does share that early 00s vibe I love so much about your sibset. It’s a bit less classic, but it’s so cute, I can’t help but root for it!
[name_u]Taylor[/name_u] - [name_u]Taylor[/name_u] is lovely, and any other day, I’d vote for it, but like [name_u]Avery[/name_u], it feels a bit too unisex to work in this set 
All said - you can’t really go wrong with any of your names!
My favourite sibsets for you though would be;
[name_u]Bradley[/name_u], [name_f]Audrey[/name_f], [name_m]Ethan[/name_m] & [name_f]Charlotte[/name_f]
[name_u]Bradley[/name_u], [name_f]Audrey[/name_f], [name_m]Ethan[/name_m] & [name_f]Hannah[/name_f]
[name_u]Bradley[/name_u], [name_f]Audrey[/name_f], [name_m]Ethan[/name_m] & [name_u]Madison[/name_u]
[name_u]Bradley[/name_u], [name_f]Audrey[/name_f], [name_m]Ethan[/name_m] & [name_f]Savannah[/name_f]
Hopefully this all made sense, LOL Good luck!! 