Do make bridge LAN1 of ether2-ether4 interfaces, assign/move current IP, DHCP server etc to it
Do make the second bridge LAN2 and assign the remaining interfaces to it and create new DHCP server, assign IP etc.
To block traffic between LAN1 and LAN2 you just need to create drop rule for forward like this:
/ip firewall filter
add action=drop chain=forward dst-address=192.168.10.0/24 src-address=192.168.20.0/24
add action=drop chain=forward dst-address=192.168.20.0/24 src-address=192.168.10.0/24
or you may use in the configuration “in” and “out” interfaces for this.