Even though you say plains game, will you be hunting in an area with dangerous game? Something to think about on caliber selection. If you want to rely on your PH as your back up gun, your choice. I’ve been to Cameroon, Mozambique, Zimbabwe (x2) and South Africa (x4) and have been too close to some cheeky mama elephants.
If in a pure plains game area it depends upon species and location. Plains game can be a 2lb Genet, 25lb duiker to a 1500lb eland. I’ve hunted KZN and other very open areas in the mountains and farm fields and you can see for miles and shots can be long but the animals are usually smaller. However, there are eland around in those windswept free range grassy areas up in the mountains. A 6 - 7 mm round would be fine in those types of environments . In those open areas I’ve used 6.5-284, 300 Wby and 375 H&H.
In Mopane forests and thicker bush area’s like Limpopo, I’ve used .22LR, 22 Hornet (small cats), .223, 12 gauge, 30-06 and 375 H&H.
In areas where dangerous game is present and I’m focused on DG, I’ve gravitated to my 375 as my “light rifle” with a mix of solids and softs. I’ve taken animals the size of bull Elephant, Cape buffalo, Serval, Oribi with the 375 and it also doubles as a back-up for dangerous game if my 416 would have a mechanical issue and go down. Smaller plains game can be shot with the solids to minimize damage. I’ve now done a few Safaris with this combo.
That got a little long winded

. My advice is to base your caliber selection on the type of area you will be hunting, size of game you are after, rely on your PHs advice on what to bring and focus on studying shot placement.