Check if you should have this line. It has been a long time I needed to use iptables for forwarding, nut I do not remember using -A FORWARD and -o together. Do you at all need all those -A FORWARD lines for masquerading?
Long time here too... But MASQUERADE on both interfaces feels wrong given what it does... Shouldn't this only be on the 'outside' of a router... Or did this change at some point in the last few years?