GL-BE9300 (v4.x) - DMZ and Inbound traffic failing with Multi-WAN Load Balancing

yeah you are right, something like this :

# create routing tables
echo "100 wan1" >> /etc/iproute2/rt_tables
echo "101 wan2" >> /etc/iproute2/rt_tables

# blueprints for gateways
ip route add default via 192.0.2.1 dev eth1 table wan1
ip route add default via 192.0.2.9 dev usb0 table wan2

# netfilter marking (OpenWrt nft syntax)
nft add table inet mangle

nft 'add chain inet mangle prerouting { type filter hook prerouting priority mangle; }'
nft 'add chain inet mangle output { type filter hook output priority mangle; }'

# mark incoming on WAN1
nft 'add rule inet mangle prerouting iif "eth1" connmark set 1'
nft 'add rule inet mangle prerouting iif "eth1" mark set 1'

# apply policy routing based on mark
ip rule add fwmark 1 table wan1