I've never seen
Belle used outside an American context. I find it sleazy, except for Snoopy's sister.
Bella people used to abound, though the youngest I've met must be in her 60s by now or even 70s.
Bella used to be OK but has gone to the dogs, pun intended.
I like
Isabel, but I like
Isobel more. I also like
Annabel and
Rosabel;
Christabel would be fine too but only for Christians, otherwise it arouses the obvious expectations. But I wouldn't use
Bella as a nn for any of them.
I prefer Mirabel to Maribella and
Mirabella, and I find
Annabella too fussy and frilly.
Annabel, then!