My personal preference is to keeps things simple, until you can't. Planning a current network design based future maybes (not hard requirements) can increase cost, complication and reliability, without any direct benifit today. So my suggestion would be to set up the simplest design first, and plug the two WAN ports into the MX. Later, when you eventually (hopefully) get a warm spare, a more complex configuration is required. Just be sure you do not move your single point of failure from a firewall to a switch.
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