I got it working with following setup:
nft add table inet partoffload
nft add flowtable inet partoffload ft '{ hook ingress priority filter; devices = { lan1, lan2, lan3, lan4, lan5 }; flags offload; counter; }'
nft add chain inet partoffload lan '{ type filter hook forward priority filter; }'
nft add rule inet partoffload lan meta l4proto { tcp, udp } flow add @ft
it excludes eth1 from hardware offload and enables sqm on eth1.
would be nice if this would be possible in luci.