starter relay issue

My guess is insufficient voltage to relay coil winding. A voltage below 10V, is likely not enough. The circuit involves, NSS, bulkhead connections, IGN2, fusible link and other connections. A voltmeter placed to measure relay coil while cranking would be helpful for trouble shooting.

Sufficient voltage there is important, when the relay coil is energized, the magnetic circuit pulls the contacts together, there is a spring that works to hold them apart. The magnetic circuit strength increases with current, and that is based on supplied voltage. A weak pull-in, results in greatly increased contact erosion.

Seems like changing relays is not the root cause of problem. Perhaps low voltage to relay coil is, let a meter be your friend.