I agree with the OP. An 18” barrel on an 870 is a very handy and light feeling firearm. And yes shoot one load one is a valid tactic. You’re having a bad day if you need more than four buckshot rounds.
I had a 2 shot extension on my 870 but it messed up the balance and feel of the gun. So I removed it. Wilson combat sells a very nice, very small one shot extension. It is like an inch larger than a 4 shot mag tube, is bulletproof, has no weird transition, no place for shells to jamb or get hung up, and barely weighs anything. I think the OP would love the Wilson one shot extension.