This is tough to debug without looking at the car and trying some stuff.
You probably have a heavy cable from the battery right to the starter.
But that is likely a custom made wire.
How does the terminal look at the starter?
How does the starter relay main stud get fed?
The ground is also a potential the problem.
If you are bonded to the frame/body in the trunk you can try adding even temporary a cable all the way to the block and see if that improves the situation.
You probably have a heavy cable from the battery right to the starter.
But that is likely a custom made wire.
How does the terminal look at the starter?
How does the starter relay main stud get fed?
The ground is also a potential the problem.
If you are bonded to the frame/body in the trunk you can try adding even temporary a cable all the way to the block and see if that improves the situation.