I’m new to guns & still doing research, so advice instead of criticism would be appreciated.

I want a compact, legal AR pistol ( CAR GUN ) that can fit in my backpack as well. I’m debating .300 BLK with a suppressor vs. 5.56 with cookie cutter for cheaper range practice.
I know 5.56 loses velocity in 5–7” barrels, and .300 BLK ammo is more expensive.

Questions:
Which makes more sense for my situation?
Are cookie-cutter muzzle devices worth it on 5.56?
Can I use a separate .300 BLK upper on a 5.56 lower?
Any legal considerations I should know about?
Appreciate any advice.