Curious, why not use a nice 22 rig? Quieter, simpler, likely easier to make good consistency etc. A 300blk would be very easy and do a better subsonic job as well.
Anyways, all that said, I love my shorty 223, so I'll not be talking you out of snagging one lol.
Lighter, often shorter oal bullet means less powder and less noise. It can be a good thing for close range plinking.
Some nice flat base bullets, rounder nose, easy to stabilize bullets would be my first pick. That shape has done well for me in many mouse fart loads in many cartridges.
A nicely coated cast bullet tends to fill this niche well if you are familiar with them, again usually a stumpy style bullet. Otherwise, may be more trouble than it's worth to you.
The pointy sexy bullets like the smk or eld can work too of course, but often the more "normal" ones will be much more cost effective. lots of testing is in order to see what your rifle likes, and you might be surprised what works best subsonic vs supersonic.
900 or 1000 fps tends to do well and be very quiet. Should be pretty easy to get decent enough consistency inside 50 yards. Longer range tends to become exponentially more difficult if you're after sub 2moa consistently.
Some reduced capacity cases could be interesting and help with the ES... but that is likely not worth the trouble.
Trail boss or tin star would likely be easiest powder to work with. Other fluffy pistol powders may work well too if you are experienced in that realm.
