I agree with other posts about the factory brakes. They greatly reduce recoil but are obscenely loud (seem to be louder than a side port brake). If you want a gunsmith to install it send it to Karl at Kampfeld customs. His brakes are very similar to a factory weatherby brake. He does excellent...
The only thing I have seen hurt the 1/2” AR500 plate is a 30-378 Weatherby shooting 180ttsx Barnes. It punches neat little holes straight trough. Even out at 300 yards.
That appears to be galled. I hope you weren’t planning on screwing a pre fit on that action. I’m sure someone can chime in that’s smarter than me or had to deal with this before, but I think a smith should be able to clean up the threads. May have to cut them larger but then a prefix won’t work...
I haven’t used 3/8” AR500 plate, but 1/2” I have. No issues damaging the 1/2” plate with my 270wsm 126HH at 3350 fps or a 6.5 PRC with 124HH at 3320 fps.
The 1/2” AR500 plate doesn’t even get boogered with a 300 Wby 200 Barnes LRX at 3000 fps.