Many-to-many NAT with overlapping subnet

We have the following setup:

Internal Interface: 172.16.1.1/24

DMZ interface: 192.168.0.0/24

DMZ 2 interface: 192.168.2.0/24

Remote office via site-to-site VPN: 192.168.0.0/16

We have to NAT the entire 192.168.0.0/16 network outbound using 172.18.0.0/16 to avoid overlap.

I.e. instead of me connecting to 192.168.x.x I connect to 172.18.x.x which gets translated as it leaves the IPSEC interface.

I setup a Virtual IP as follows:

Interface: IPSEC-interface

External IP Address/Range: 192.168.0.0 - 192.168.255.255

Mapped IP Address/Range: 172.18.0.0 - 172.18.255.255

This causes problems with our connected networks however, 192.168.0.0/24 and 192.168.2.0/24.

When the NAT is activated we lose connectivity to the directly connected 192.168 networks.

Would appreciate any insight on if/how we can translate the traffic ONLY if it's going out the VPN interface.

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