Post a network diagram so we can be sure what we are talking about.
I guess when you set up the OpenVPN server (3) you put all the local subnets (2,4,5,6,7,…) in the "Local Subnet/s" box. Or you are redirecting all traffic from clients to the OpenVPN.
Do a traceroute from and OpenVPN client to subnet 5 - that will show where the packet is going (around in a loop somewhere maybe).
If the router inside your LAN (that routes from 2 to 4,5,6,7...) is blocking traffic originating from OpenVPN (3) tunnel network, then why not change that router config so it passes the traffic?
Otherwise, yes you can add an Outbound NAT rule on LAN that will NAT traffic with source "OpenVPN tunnel subnet" to the pfSense LAN IP. That will hide the OpenVPN tunnel network addresses from the inside router.