Thoughts:
10.5" barrel will provide all the longer-range performance you need and be more reliable without tuning. Go below that and you want to make sure the gas system is tuned to the shorter length. I believe that's just a matter of opening it up a little but don't quote me on that. You should get pretty much all the oomph out of a cartridge and can run at least up to 62gr with a 1:9 or 1:7 twist (memory may not be correct about 1:9 and that weight).
Shorter barrel is shorter length, of course, but you will lose some speed and even with 1:7 twist it's best to keep round weight below 60gr for best performance. I have a 7.5" barreled pistol that is my 'interstate bag gun' that I consider to be a 150-200 yard weapon max. It'll still leave a mark farther out I'm sure, but velocity suffers at the rate of approximately 100 fps per inch, IIRC, and running 52-55gr rounds I prefer to keep it close.
Arm braces be good. I have KAK shockwave but want to at least try the brand that I can't remember the name of right now--HATE it when that happens...
LAW folders good to reduce length for storage and carry. LAW folders not good because at best you get one shot before unfolding them, which might make it to slow to get into action in fullness. I have a covert bag that allows me to get away without one.
You might well find that the AR pistol doesn't gain you as much shortness as you thought. It's advantageous for other reasons, however. Here is a picture from the fourth book in my 'Gunfighting, and Other Thoughts about Doing Violence' series that has some comparisons. I have a big chapter on C-PDWs (Civilian-Personal Defense Weapons) in book, which category I put AR pistols into. (My AR pistol is 3rd one down in this photo.)
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