Good to see ya back Tyro.Feel free to E me if there's anything you'd like to say privately.
First, one big need in WIHTF hardware is consistency. One doesn't need to stop and thing about where the slide release is, or if they have to pump it, when life and the universe shrinks down to one very small area of engagement, where life itself is dependent on fractions of inches and seconds.Since you've started with the 870, stay with it. Also, while it's your life and your family's, there's little that can be done with a 12 in your case that can't be done with that 20.
But if you just WANT a 12,that's OK too.We understand(G)...
As for semi autos, there's some good ones out there. However, I can get repeat aimed shots off as fast or faster than many autos can cycle, including the time-honored Auto 5 Browning. Any good pump gunner can.
The scenario you mention may be solved by shooting left handed. I recommend that all serious A-S shotgunners practice that way a bit, for there may be times when it's the best option.