I’m a southpaw and I’ve never found a bottom eject to be any more left hand friendly, I own a couple Ithaca’s. You can’t drop a shell in quick and port load a bottom eject shotgun, I’ve never been hit in the face with a shell on a side eject shotgun either. I’d rather have rh pump guns or semi auto rifles, as your support hand is on the right side to rack the bolt or port load a couple quick rounds with having to reach under/over the receiver like a rh shooter would. Your on the rh side of the gun so looking into the port and seeing an empty chamber makes it easy to clear and prove it safe, I’ve never had dirt interfere in function so bottom eject isn’t a huge deal for me.
The problem with LH 870’s is there is next to no extra barrels available for them, I’ve only ever seen 28” vent rib barrels being sold. Which is the one thing that is great about an 870. I have 12.5”, 14”, 20”, 22”, 28” barrels for my 870’s and clones, you can’t find different barrels for a LH model. Kinda defeats the purpose of buying an 870.