Thank you all for your help, this information will prove very helpful for me in the coming months when I will start experimenting with the big 375.

neck sizing only sounds like a very good idea with this belted magnum, as does checking length after every firing. I have little experience with annealing cases, did that with my 45-70 with some success (reduced case sootiness somewhat) but never with anything high pressure.

I just got my reloading supplies, dies from Hornady, brass and 300 grain RN bullets from PPU. According to my experiences PPU brass and bullets are not super high quality but absolutely serviceable, I use them for my 7x57, 8x57 and 7.5x55 with irons all the time with respectable results.

As for loads, right now I have only one suitable powder for it, that's Lovex S070. Since pretty much every shooting range is closed due to the annoying pandemic right now I will send the loads to the proof house instead and let them analyze pressure, speed and consistency of my reloads.

My first handloaded .375s. I chose an OAL of 3.527 - at 3.543 the projectiles start to kiss the lands. I cycled them a couple times to check feeding. Have to use non-magnum standard CCI primers though, due to the current events the rarer magnum rifle primers are very hard to find.
Maybe the non-magnums will work well enough, I had a .300WM once that absolutely hated magnum primers and would only group well with standard primers.

Also - fun fact - the PPU 300 grain round nose bullets have the exact dimensions of a 124 grain 9mm round, you can even load the projectiles into a Glock 9mm magazine. Things you find out when you are fooling around with reloading supplies.