/ip firewall mangle add chain=postrouting action=change-mss new-mss=clamp-to-pmtu passthrough=yes tcp-flags=syn protocol=tcp out-interface=all-ppp log=no, тогда микротик сам будет дробить пакеты правильно
/ip route add dst-address=0.0.0.0/0 gateway=1.1.1.1 routing-mark=ISP1
/ip route add dst-address=0.0.0.0/0 gateway=2.2.2.2 routing-mark=ISP2
/ip route add dst-address=0.0.0.0/0 gateway=3.3.3.3 routing-mark=VPN
/ip firewall mangle add chain=prerouting src-address=192.168.0.2 action=mark-routing new-routing-mark=ISP1
/ip firewall mangle add chain=prerouting src-address=192.168.0.3 action=mark-routing new-routing-mark=ISP2
/ip firewall nat add chain=src-nat action=src-nat routing-mark=ISP1 to-addresses=1.1.1.2
/ip firewall nat add chain=src-nat action=masquerade routing-mark=ISP2