Not able to link portals properly

As far as I know, it works like this:

When entering through a portal from the overworld, the corresponding coordinates in the nether are calculated - that's x/8 and y/8 of the overworld coordinates.

then the game checks whether in a 128 block radius around these coordinates, there is a portal in the nether - if there is, it will take you to the closest.

So, if you want to portal from the overworld to a netherportal (let's call it playerportal) that is close to your "kill spot", but not quite there, build that playerportal (in the nether) where you want to have it, then take the coordinates, multiply them by 8, and build a portal at that spot in the overworld.

Please note, that this might initially break the mob transport - if you have multiple overworld portals to transport the mobs to the kill spot, and some of them link to a point in the nether that's closer to your playerportal, the mobs will go there. Make sure the Portals are spaced correctly. You might have to do some math.