Personally, I would deploy StorngSwan on Ubuntu at your hub location (because it is free and excellent). Build the IPSEC VPN from that to StrongSwan. Then StrongSwan and your hub MX only need static routes between each other.
Failing that, I would buy and run a Cisco vASA at your hub location.
Another option would be to buy something like an MX67, run it in VPN concentrator mode, and ask the remote party to host it at their location. They could then add a static route on their firewall to the MX and treat it like an MPLS WAN router.
Failing that, you could buy a little Cisco Firepower 1010 with ASA software and put it at your hub, and dedicate it to terminate non-Meraki VPNs. You would have static routes between your MX and Firepower.
No matter which solution you choose, my recommendation is to use an additional device (either virtual or physical).