Keep in mind that the flow preferences are just that: preferences. In case of issues with one of the WAN links it will still failover to the other. Imo there's no way to do this, at least not with the default features available in dashboard. Unfortunately 1-1 NAT also doesn't help as source IP for outgoing traffic is still NATed to the MX IP. If I were you I'd reach out to Meraki helpdesk to see if any hidden feature could be enabled. If not, definitely make a wish.
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