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What gauge are the battery cables? Where does the ground wire terminate? Is there a heavy ground strap between the engine block and chassis?
There could be internal corrosion on the cables at the terminal connections causing resistance.
Could you connect the battery to the relay with another cable as a test, even if you have the battery out of the car temporarily as a troubleshooting step?
Something has changed over time. If you changed out the starter, relay and battery with all known good components, the items left in the equation are the cables and ground connectivity.
My money is on internal cable corrosion. Hard to tell unless you cut away the insulation at a terminal, then you'll usually see green on the copper wire or the white powder corrosion inside the terminal.
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