I hold the pick slightly different when strumming. I guess you could call it a "rock and roll" type grip on the pick where instead of the 1st finger running parallel with my thumb (last inch from nuckle) I instead position the 1st finger so that the pick is more towards the Tip of the finger...so in essence I have "extended" my first finger vs having it bent at the first two knuckles.
Confused yet? I may take a picture or tw0 and post them.
Also, on the down stroke there is "no give" on the pick. On the "up stroke" however the angle changes....almost to the same angle as one would use on a "rake" and is more or a looser grip, which the 1st finger provides support so I don't loose the pick.
I also "flip" my hand with a loose wrist. Don't get me wrong, the bass notes on the down beat are hit just as hard "driving through the string" as I would when flat picking, but the up strokes are a lot less severe.
Only problem or issue I have is switching from flat picking to rhythm and from rhythm to flat picking...it takes a conscious effort on my part during the last measure to start changing the pick/finger position in order to make a smooth transition.
On thinner picks....sure that would help you, but man oh man I hate that "clicking noise" in rhythm guys that use a thinner pick gets....drives me nuts! I use a real tortoise pick and I guess it's about a 1.4 and pretty damn stiff (I've never felt it "give") If I drop down to a 1mm then there is some "give" but it starts clicking like those African tribes that click to each other to communicate.
EDIT TO ADD:
Also at say 220 bpm -300 bpm I do not hit "all the strings" in my strumming. I am more or less concentrating on 3 or 4 strings and making a "cross picking" type motion for the up and down strokes...I focus mainly on the bass note of beat 1 and do a cross picking type move after that.