Whoops, my bad!
The problem with Google Contacts for instance is that it lists all my 10000+ customers (who are consumers, not businesses) who have ever emailed us. It gets kinda messy. Also every staff member here has their own @company email address, thus a different addressbook right? (We use Gsuite internally)
Also I don't see how a IBAN is connected to a Google contact. I'm not quite sure what the solution is when you don't want to you external servers (understandable), but I think manually entering an IBAN every time we do a transfer will not be appreciated by our staff. I'm willing to help you testing if you know any solution!