I would recommend instead of buying untested Ruger polymer GSR mags that you get AI/AI clone mags that have been tried and tested.  I know that the polymer mags are half the cost of the AI/AI clone mags, but I will say the AI/AI clone mags are the industry standard for DBM systems.

Nothing wrong with having some polymer mags since you hunt with them and they offer a 3rd flush polymer mag, but for SHTF, AI/AI clones should be your go to mag.

If your state allows NFA, I'd look into a suppressed option before another firearm.  A suppressor will help you out more than another firearm.

Rifles always cost more compared to a pistol since you need bases, rings, optics, suppressors...etc etc.