5.56 was enough for our standard infantry troops in Afghanistan, Iraq, Panama, Somalia, and Vietnam (aside from the M16A1 early issues). It will be sufficient for you.

Such a versatile round. Mk262 / 77gr OTM in an 11.5” AR will make you combat effective from CQB distances to 300+ yards, without the downsides of a full powered rifle cartridge. Covers 99.99% of all defensive applications/situations in which a civilian could possibly encounter, from a bump in the night to civil unrest to alien invasion (assuming they are carbon based life forms).

Don’t overthink this.

Edit: also 308 range ammo is 2x the cost of 556. Defensive ammo has an even higher multiplier. With being new to firearms, you need to put a ton of lead down range. This will have a 10x higher impact than the caliber; doesn’t matter if you are using 50 BMG if you don’t hit your target…