I take it you checked that the new main relay had fuses fitted, they don't always come with them? Sometimes fuse holders and terminals are coated to prevent corrosion in storage which needs to be removed/rubbed off. Maybe try plugging in and unplugging the fuses/connectors several times?

Have you tested the new battery? I've bought batteries in the past that have been faulty out the box - it might show 12.5+ Volts on a multimeter and fool the Optimate into indicating it's good but as soon as a load is applied that quickly drops to zero as they can't provide any current (Optimates don't perform a load test, only no-load voltage drop).

Are the battery terminals clean and tight?

Have you dislodged a cable when removing/replacing the battery?

Could be the ignition switch? You might not get ignition, but if you turn it to the park position, do the side lights work?

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