Perhaps the Woodleigh PP bullets, 500 or 400 grain, could solve a lot of the feed dramas in these rifles. PP's were not available when I was using my Brno .458, so I never had the chance to try them in it.
They have a spitzer ogive with no exposed lead at the nose, so it should be way easier to get them to feed nicely, compared to the round nose profile bullets.

The .458 is capable of really flattening the exposed lead on round nose projectiles (in the magazine) from recoil, and this can create more feed problems, that might not show up when using dummy rounds.
Also the rounds in the mag can/will move forward from recoil in that long magazine.

A spacer at the front of the magazine which keeps the cartridges to the rear, plus the use of PP projectiles, plus some fine tuning of both the follower angle and spring tension, should get things working properly.
I'm sure you will get it sorted out soon.
Best of luck with it.