Completionist players in a sandbox

[nedz]

There comes a point, in some combats, where I just hand wave the result.
If the result of the combat is a forgone conclusion, such as the cases you describe, then it is fairly pointless running through the numbers.

Ed: one thing you can do, sometimes, is to have reinforcements turn up. Now another 400 CR 1/2 goblins is not going to cut it, but a tribe of Hill Giants, say, just might. Just have the sound of the fighting attract a 'random' encounter.