I've used it at a client and it was as simple as setting up the third party vpn with a 0.0.0.0/0 remote subnet listing.
You seem to have resolved the one problem you were having, is there something specific you are looking for insight on?
This wasn't available when I used it, but you may want to look at VPN exclusions for certain known traffic to optimize performance: https://documentation.meraki.com/SASE_and_SD-WAN/MX/Design_and_Configure/Configuration_Guides/Site-t...
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