Help. Would like to buy some kind of starter assualt rifle and need opinions.

To start, I know nothing about them. I understand you get what you pay for, but Im not into spending thousands of dollars.

With the current climate of this country, I decided I would like to add something to my safe that can shoot a bunch of bullets quick, that I can stock up on bullets for a decent price.
Optics wise, I just want something basic, but not junk.

I have high powered bolt action rifles, shotguns, revolvers, muzzleloaders, semi auto pistols, semi auto 22s, lever action 22 mag, H&R 444.

Guess what Im looking for is a semi auto rifle like an AR15. I would like something with a longer range than a pistol, with the capability to change large capacity magazines and shoot semi auto. Also would like to be able to buy bulk ammo for a decent price so that I can stock up as quickly and inexpensively as possible.

Im not a hobby shooter, just want to get setup with one of these rigs due to all of the craziness going on, and the possibility that one day these guns and the ammo are harder to get your hands on

I've seen a Keltec Sub2000 9mm rifles that take Glock 17 mags for $399, without optics. Im interested in that, but know nothing about it. Is 9mm something you can stock up on at a decent price?

Opinions wanted, and thanks for the help.