IPv6 internal network is simpler and probably a good idea for a brand-new network.

The big advantage is that don't have to worry about subnet size. No deciding how big subnet is going to be, and either making it too small and having to resize or making it too big and wasting space.

IPv6 is more complicated in that supports multiple addresses, but that is an advantage. For internal use, assign ULA addresses and route those over VPNs. For accessing Internet, computers use the ISP assigned addresses. Then assign fixed addresses from hosting provider to load balancers and external servers. This means that only Internet only sees random addresses; they know the provider but that is known with IPv4.