The reason the striker needs the bushing (or pex, etc) is the latch is designed to work with the diameter of the bushing.
If you study the latch and the striker you'll see how and why being bushing-less creates lots of issues.
The striker needs to be able to latch the latch through both clicks, and once latched, hold the door tight to the weather stripping. These actions require both sides of the striker, one side is responsible for latching the latch, the other side is what holds the door tight to seal it up.
So if you reduce the diameter of the striker, like when it's bushing-less there needs to be more travel to be able to make it function properly. Issue is with the weather stripping in the way it requires a good slam to get that extra travel, then the door is still going to be loose.
So begins the adjustment attempts, but the actions are inverse of each other. By moving it in or out you are compromising the inverse action.
Basically, you need the bushing, so buy a new striker or do the PEX trick to your old one instead of slotting holes out or trying to beat the flange of the cab because you will just chase the issue around.
The bushing also provides support to the rear of the door and should take a little load off of the hinges.
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A bushing less striker
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New one
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