Re: Too much pressure in the 300 RUM?
The other 'best' way to monitor pressure is with a chrony. If the velocities are within reason, odds are pressures are too. If you are way above (say over 5%), you can bet pressures are toasty.
With regards to primer pocket life, if your pockets get loose in under 4 reloads, your loads are very toasty. That is where I was running my RUM. Dropped loads a bunch and now am on my 7th reload with no issue. Primers looked the same after firing so don't really pay much attention to that anymore.
I also neck size so bolt lift will get harder all on its own. Again not that accurate of an indicator unless you need to hammer it open, then...
I shoot in BC so know all about the cold weather. Nice thing you are using Retumbo. It should not vary much when temps go up to +30C. Always start low and verify but I have shot the same loads in some very hot weather with little change either in POI or accuracy.