VC
I've run Quickload using Somchem S365
71gn (lightly compressed) using the Hornady 286gn gives 2577fps at a guestimated pressure of 50KPSI.
64gn Somchem S335 with the same projectile gives 2526fps for a guestimated pressure of 63KPSI.
Shows that S365 is too slow & S335 too fast - but whatever you can get in SA might be all you can achieve!

64gn Norma 202 gets 2583fps for guestimated pressure of 62KPSI (worth a further look at)

When I tried 67gn Vihtavuori N540 with the Hornady 286gn it gives 2578fps at a guestimated pressure of 61KPSI - still a bit fast I think but still a good contender.

However!
73gn Vihtavuori N550 (lightly compressed) with the Hornady 286gn gives 2693fps at a guestimated pressure of 62KPSI.
This was the real surprise!

I would definitely try this as the best performer as to what you can get in SA!
We can't get this powder here in Oz, but if we did I would certainly give it a crack!
Take all precautions going up in small increments as this info is only a "guestimate" but it should point you in the right direction & give you a basis to work towards.
I wouldn't use magnum primers here in this case as powders required are of the medium type only.

By the way I ran your load of 54gn of N550 & it can back with 2036fps for approx 26KPSI.
All these loads have been calculated with a 26 inch barrel - so allow approx 25fps less per inch of barrel.
So rather close indeed!

The 9.3x64 can get well over 2600+fps for a 286gn pill & a solid 2570fps for the 293gn TUG, but you have to use powders that suit the cartridge/bullet combination to achieve the best ballistics for a given maximum pressure; but this must also be on the proviso of accuracy as well.
Some powders/bullet/primer combinations in your rifle might get the velocity, but be like a shotgun & spray them everywhere - lets hope not for N550!

I hope this helps you in your endeavours - go slow & stead & watch for pressure signs.
Best of luck mate, keep us informed
93x64mm