Nope, MX does not support any STP mode.
The MX does not run STP in any capacity, and will not exchange BPDUs with other switches or participate in the root bridge election process. If the MX received BPDUs on the LAN, these BPDUs will be re-forwarded within the broadcast domain that they were received on. If there are multiple switches connected to the LAN of the MX participating in an STP election, all BPDUs sent to the MX will be forwarded to other links with the same VLAN allowed, which can cause switches to see BPDUs from multiple other switches, causing ports to get into an unknown/unidentifiable state and impacting the root bridge election process.
https://documentation.meraki.com/MX/Networks_and_Routing/MX_Layer_2_Functionality#Spanning_Tree_Prot...)
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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