Add a rule at the top of your firewall policy accepting all traffic between the subnets in question, then force the traffic into the fastpath where there will be minimal further enforcement:
sk156672: SecureXL Fast Accelerator (fw fast_accel) for R80.20 and above
It isn't quite acting as "just a router" but it is pretty close. Only catch is if the traffic is in the slowpath/F2F it cannot be forced to the fastpath using this technique.
Gateway Performance Optimization R81.20 Course
now available at maxpowerfirewalls.com