Any .223 is going to be loud enough, even with a suppressor, that you will need to still wear ear protection. Barrel-length will make little difference.
A suppressed .223 sounds about like an un-suppressed .22 magnum.
If you shoot it in a confined area, one shot will make your ears ring.
If you want to run "sub-sonic" .22 loads, consider getting some Mexican "Colibri" ammo and a cheap .22 bolt gun.
...and yes, suppressed AR's make a dandy mess of your trigger group after only a few shots. Most of the mess comes right back down the barrel. So, the fancy-schmantzy "piston gun" won't help with the mess.
Unless you shoot on a windy day, you'll get a big un-breathable smoke cloud around your face too.
If you take it on-and-off the rifle regularly, its just a matter of time before it goes back on slightly caddywampus, and you get this.
Same thing too if you let others shoot "their ammo" in your rifle,
...which costs this much to fix.