Your kids’ names are fun!
Here are my thoughts on your list-
Ramona-Ooh, I like this w/your crew! Kind of bold, feminine, well known yet unique. I think it’s good on its own, but the nn [name_u]Romy[/name_u] works w/Max & Sephie really well.
Maria-This is good w/Maximilian & [name_f]Persephone[/name_f] too. I don’t like [name_f]Ria[/name_f] though. It seems like a not very natural sounding nn.
Nina-Meh, this doesn’t do much for me. It’s okay though.
Adeline-I absolutely love this name on its own. [name_f]Addie[/name_f] names are just so trendy though, unfortunately . And [name_f]Ada[/name_f] seems like a bit of a stretch for a nn here. [name_m]Just[/name_m] [name_f]Ada[/name_f] is good, even if it is a bit short w/your other kids’ names.
Here are a few more ideas-
Hope this helps.