That "preventing traffic between specific VLANs" will be done properly with filter forward rules, as it should be.
The action=accept chain=prerouting rule in the mangle table only acts within the scope of the #4 box of your flowchart. This action just skips the rest of the mangle chain=prerouting rules, processing will continue with #5. It doesn't means skipping the checks in #14.
Even in the filter table, action=accept chain=forward just means skip the other filter rules of the box #14, and go to the next box, which is #23. It doesn't means skip the rest of the firewall (#23, #24 and #25).