Disable reset/debug menus on boot

I'm still getting the screen with: ‘ ===>' if the user holds down the button on start up...

Yes, that's expected. The === bit is the bootloader - replacing that feels like a really bad idea, but if you do the other stuff then as long as they don't flatten the battery and hold the button while applying power, they will never see it.

Should have commenting out recoveryMode = true; prevented the ' ===>' screen from appearing (on startup)?

No - it's just the recovery menu.