I think you are wise to not assume the cause for your symptoms and even wiser to revisit the basics (clean all the connections).
The VAST majority of electrical failures are connection failures.
You spelled it out well - corrosion is difficult to prevent. Even a well-clamped connection can loosen up from corrosion and thermal cycling. Connecting to/between highly corrosive materials (steel, zinc, tin are examples) will be more likely to fail than clamped joints between copper, nickel, and noble metals.
The best approach to solving any problem is to define it. If it were me, I'd be measuring voltage drops along the circuit before spending money on new parts...
For example, measure the voltage across that disconnect switch while cranking the starter so you can make an informed decision.
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