One point - if you remove (ok, disable for the beginning) the following mangle rule:
add action=mark-routing chain=prerouting dst-address=192.168.88.0/24 new-routing-mark=vpn
you can also remove this route:
add distance=1 dst-address=192.168.88.0/24 gateway=bridge routing-mark=vpn
However, the critical point here seems to be how the OpenVPN server at the remote end is configured. Unless it is another Mikrotik, it is not enough to indicate in its system routing table the 10.11.12.2 as the gateway to 192.168.88.0/24, you have to tell that separately also to the openvpn process using the client configuration file.