Your suggestion re: scapula is familiar to me. I suspect as you probably already know my issue was probably 90% psychological.
Muscle memory, path of least resistance, etc. I got into power lifting in my 20's. At that point I had already become too dependent on using my deltoids. Given the right circumstances and in enough time, dumbbell flies could do the trick a little bit. Water under the bridge at this point.
My goal these days is good bone density. No point in living to the ripe old age of 150 years if I have brittle bones and can't hike in the woods on a sunny day .
But yeah..I would put a few days between squats and dead lifts days if your goal is muscle mass.
Strength exercises are not always the same for gaining mass. I suspect you already know this.