I would fix / check the DL question first. Many, most, archers when really checked have too long a draw. Not me talking but several experts. Just because you are used to this DL does not mean that is correct. IF not absolutely sure check using one or more of the procedures outlined in threads on AT, or the Archer’s Friend website, or Bernie’s book, etc..
Also, I have read where many bows actual draw is different than published, could this be part of it? Have the actual draw checked. Could be these bows are on the long side? As a question...
The reason I focus on DL, as it is a significant reason for slap. It was in my case and I was in the fifty first state, (The state of denial), for about 1 year… At 6’1” I just HAD to have a 29” draw… even the pro-shop owner told me I was… However, once I actually measured!? It said I should be 28 to about 28.5”. I am now shooting 28” and never have hit my arm since! May try 28.5”
Also, even if you learn to overcome the slap, by adjustments, a too long draw has other, more subtle, downsides.. Like causing bad form, that causes you to be less steady, that can cause TP, etc..
I, personally do not agree that an STS should be used to allow a too long draw.. My opinion. Like the STS, but not for that.
Bernie, (And yes he is opinionated but that does not make him wrong), says that until the proper DL is established there no reason going further in form, etc.… Strong statement.. I tend to agree. again, my opinion.