OK, here’s a bizarre, random thought. What if you took a Para TTR upper, cut it down to just in front of the hand guard, leaving about a 10.5 inch barrel (if my info is correct), and re-thread for a flash hider/suppressor mount. This would eliminate the need for a receiver extension. I’d rather have a 7.5 barrel, but this may be an acceptable compromise yielding about a 3 inch shorter overall pistol.
ZM Weapons (what it used to be before the YHM manufactured version and now the Para TTR) used to offer an 11.5 inch upper… so I would say that your idea is definitely doable, as long as you can find someone to cut the barrel and adjust the system for you.
Sounds like it would end up very similar to one of those Olympic Arms OA-93 pistol jobbers. Haven’t seen one of those in a long time…