I have a really nice recoil pad on my current LR rifle and it absorbs most of the heart rate bump. With my other rifles I had to reduce heart to 64 or less and plan a shot between beats, or pull the rifle off my shoulder to reduce bump. I try and get as much bone on stock contact as possible as well, or use a shooting vest that is designed to reduce bump.
For me, if I have time, like at a range, I take 5 deep solid breaths and the go to shallow breathing, exhale softly, squeeze. Sometimes I go ahead and hold my breath momentarily. I try, at least most of the time, to keep myself oxygenated so that might mean a do over on deep breaths. Too much shallow breathing and you will panic breath, shake, cloud out, or just be uncomfortable.
I dunno, I generally feel OK with my hold. I have a 204 that I practiced with for a couple months at 300 and was consistenly shooting .75 inch groups. That rifle is dead on so any goofs is all on me. I stretched it to 500 and I was good for 1.5" groups. That scope has big cross hair and no parallax adjustment. I notice a tad more anxiety with the 6.5-284 tho. Perhaps the bigger bang messes with our heads more than we want it too.