Sorry to be that person but I wonder if you need a fourth option? I love your middles!
Alyssa is too dated for me and I think it makes Declan sound 90s to noughties (whereas Emily keeps him more classic and timeless). The nineties & noughties names will come back in but I wonder if given Emily was also very popular then (or the 70s-80s), you could go for a name that’s of this time but also bridges classic (eg Anna or Anne/Annie).
Elsa is cute but I’d prefer a non-E name if possible.
Emily and Amelia could be two Emmys/Millies/Lilys and far too close.
How about:
Anne
Anna
Ailsa
Alexis
Alicia
Abigail
Elysia
Alessia
Cara/Kara/Cora
Carys
Heather
Annabel
Madeleine
Annaliese
Frances
Leonor
Louisa
Laura
Odette
Sarah
Clara
Chloe
Ivy
Iris
Stella
Serena