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No, I don't think it's too modern, I actually think quite the opposite - I live in Europe as well and I think that it could be seen as quite a vintage name x
It's a really difficult situation with the differences in the popularity between Maria and the other names you have for your children. Maybe try adding a more rare middle name in order to keep the theme going - however I think Maria is a beautiful name - and if you like it you shoukd go for it!
On alternatives to Isis - Maybe Iris or Ines
Elizabeth - Elsie
Harriet - Hattie
Esmerelda - Esme
Charlotte - Lottie
Francesca - Fran/Franny
I was also obsessed with the name Oliver, that name has now been replaced with the name Elliot(t) - which I think could go well with your Liam because of the 'li' sound - ultimately similar, but not the same.
Also Luke could go quite well with Liam - still quite similar, which I think could go well together too.
Hope your adoption goes well!
Not really sure if our naming styles are the same, however here are my top 5 for each gender:
Elsie (possibly short for Elizabeth)
I hope this was helpful & Congrats on your twins!!