Bidirectional PIM is the best fit for this multicast design because the application is many-to-many and has a manageable but meaningful number of multicast sources. Cisco describes BIDIR-PIM as a multicast mode designed for many-to-many applications where sources and receivers can be widely distributed. Unlike classic PIM sparse mode, BIDIR-PIM does not build source-specific shortest-path trees for each active source.
Instead, traffic uses a shared bidirectional tree rooted at the rendezvous point, which reduces multicast state in the network and improves scale when many sources may send to the same group. Source-Specific Multicast is optimized for one-to-many distribution where receivers know the source and join an (S,G) channel; it is not the natural many-to-many choice. Any Source Multicast can support many-to-many traffic but usually creates more state because source trees may be built. Multicast VPN is a transport/service architecture, not the specific multicast mode requested. Reference topics: BIDIR-PIM, many-to-many multicast, shared trees, RP- based forwarding, multicast state scaling.