AutoVPN is the "proprietary" way of establishing VPN tunnels between Meraki devices. In your case this is not what you need. You need 3d party IPsec VPN, called Non-Meraki VPN in the dashboard. To configure it you'll have to set the Z3 to hub (otherwise the Non-Meraki VPN settings don't appear). You can then set the "Use VPN" column of the local subnets to "No" as you won't actually use AutoVPN. Then you configure all your IPsec parameters. Something like this:
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