No, the MX will still send its own dashboard and control plane traffic over the WAN connection, even if you add a default LAN-side route in Addressing and VLANs.
Static routes configured in Addressing and VLANs only apply to LAN traffic. They do not influence management traffic generated by the MX.
Therefore, you can safely create a static route 0.0.0.0/0 pointing to an internal next hop to direct client traffic elsewhere, the MX itself will still use WAN1 for dashboard connectivity.
MX Routing Behavior - Cisco Meraki Documentation
But be aware that you may receive a network overlap message.

I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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