I suggest that the way ahead is for you to buy a copy of 'Africa's Most Dangerous' by Kevin Robertson. There is a man who knows what he is talking about (and won Rigby's inaugural 'Dagga Boy' award). He makes various points that stick in my mind: that the buffalo is a pachyderm (literally: thick skinned); that every shot you fire (even in practice) is the equivalent of sending a bottle of Scotch down range; and that once you squeeze the trigger on a buffalo one of you is going to end up dead.
There is copious literature on the ability of the Cape buffalo to soak up punishment, including heart shots, and still hand out some righteous vengeance.
The unfortunate reality is that the 9.3x74R - I have a Chapuis double rifle in this calibre, and I prefer it to my H&H .375 - is underpowered for a buffalo. Mine is a lovely rifle, very accurate and low recoil: but it is only suitable for plains game. (Neither is a 9.5x57 M-S properly suitable.) Doing the rounds on WhatsApp three or four years ago - before Covid - was a video of two enthusiasts who went after a buffalo with a 30-06 - it's probably linked somewhere on this website. The expedition didn't end well and the video ends with some loud screaming. I imagine that one of them, if he wasn't killed, ends up in a wheelchair and/ or with a colostomy bag.
In my opinion - only - it is entirely possible to kill buffalo with a smaller calibre, but that requires either a wholly accurate brain or neck shot. If it is not an accurate shot, someone is going to have to follow it up, with the considerable risk to the trackers and PH (and, I hope, the person who took the original shot: I'd certainly consider it a bit cowardly to fluff the original shot and then bugger off back to base camp leaving others to clear up the mess). There is a moral aspect to this: if one of the trackers get injured on the follow-up, that's down to you.
Caveat: I have shot precisely one buffalo, with a .470. It soaked up 3 shots of mine, and various others from the hangers-on - .375, .416, and .450, not all of which connected. My experience left me with a healthy respect for the animal.
In answer to your question, I am afraid that I certainly think that a buffalo rifle begins with a .4. Why not start with a .404 Jeffery, which is a (comparatively) gentle round, and can be used on plains game as well? Avoid the .460 Weatherby (harsh recoil).