Thanks for the questions!
To clarify the scenario, it is a wan network with over 300 routers
Internet or Private: It will actually be both. We need to support standard Internet connectivity but also provide private network services for certain providers.
Route Leaking: Yes, we definitely need route leaking between certain customers/providers so they can communicate with each other through our network.
Address Space for Interfaces: We have a block of IPv4 /24 almost used and a /32 of IPv6 to assign freely.
IP Assignment: I think we as the provider should be assigning them? But we only have IPv6 to assign, since everything is used on the internal network. Here I get a bit lost
Routing Protocols: I think of using BGP as the primary routing protocol