Okay. So have look at this and this thread, and come back if you get lost.
You seem to have a RB model with two switches like 2011, so the ether2 - ether5 are on one of them and ether6-ether10 on the other, so the path between the two goes via the CPU anyway. So you can create two “/interface vlan”, one with “name=vlan-pppoe” and “vlan-id=100” and another one with “name=vlan-dhcp” and “vlan-id=200”, both with “interface=bridge6-10”. You then assign an IP address and DHCP server with pool and network to interface vlan-dhcp, and make vlan-pppoe a member port of bridge2-5: “/interface bridge port add bridge=bridge2-5 interface=vlan100”.