ALEX,
you are in a right royal bind!
However if I was to choose for a "African plains' rifle then perhaps .308 Norma Magnum?
This is a step up from the classic calibre of the 7x57mm but less than Group 1 of the .375H&H.
Different from all the .300 Winchesters / Wetherbys etc, better to have something different as your list is not the common types (.222 Rem for starters!)
There are still plenty of components for casting & reloading either for reduced loads or full throttle loads in .30 cal range.
But.....
If you chose say a .45/70, this could be used for the normal black powder cartridge BGR event.
As this uses the same projectiles as the .450NE, casting is then an easier option as you already (may)have(most probably) moulds for the .450 - or you could double up this way for both if you don't!
I use my .500NE for both group 3 & BP express events in BGR this way - but cast PP's only for the BP event.
Either way its a tough call for you, but it's your decision as to what you will use it for/on.
A tradesman always uses the 'right' tool for the job, yes sometimes the only one available that you have may be a little too big or too small for that specific need; but is there anything wrong with building up the 'toolbox'? I very much doubt one bloke (or lady) here would argue against that if being in you predicament!
Lucky bugger
93x64mm