Elephant in the room is the need to regulate in a double rifle.

Only way to get anywhere near buffalo capability with this cartridge is by using Nitro-for-Black loads with premium bullets. Recommendations for N-for-B loads that regulate in DRs, as listed in Graeme Wright's book mentioned above, is a good place to start.

If the rifle was a single-shot (or a Certus??) then you would have had a lot more leeway.