Great - a total misinformation debacle.
The 458 (or any other big bore for that matter) generally suffers less throat erosion that almost any other type of nitro powder rifle. If you don't rust out the barrel or otherwise damage it somehow, it will keep shooting as well as it did on day one for a very, very high round count. Certainly more than your wallet will thank you for.
In terms of open sights loads, I would load something like the Woodleigh 350gr BPE bullet at 2100 ft/s with H4895, which works well for that sort of reduced load. Just start at 60% of book max and work up until you hit 2100 ft/s. It should be good to about 200y before drop gets bad, which is plenty for normal iron sight use.
https://imrpowder.com/wp-content/uploads/2016/11/h4895-reduced-rifle-loads.pdf
