2 Gbps WAN/LAN NAT Routing on ramips MT7621 devices

This is a very good point. Let me start with explaining the bridge offloading feature. This is not related to software or hardware offloading at all.

Quoting from OpenWrt 22.03.0-rc4 fourth release candidate - #91 by arinc9.

There's also this feature called bridge offloading. Most DSA subdrivers implement this. The only one I know that lacks this is the rtl8365mb driver. What this feature does is it offloads traffic between switch ports. Forwarding frames between switch ports will be offloaded to the switch hardware so they don't go through the CPU and exhaust the link to the CPU.

You get full speed on switching between lan0 and lan1 thanks to this.

When lan0 is directly connected to the CPU, you can't benefit this anymore. Hardware offloading will help but it's a layer 3 feature, meaning, packets between the interfaces must be routed.

This is the rule for hardware offloading:

iptables -A FORWARD -m conntrack --ctstate RELATED,ESTABLISHED -j FLOWOFFLOAD --hw

Related patches to the FLOWOFFLOAD target:
openwrt/650-netfilter-add-xt_FLOWOFFLOAD-target.patch at master · openwrt/openwrt
openwrt/800-flowoffload_target.patch at master · openwrt/openwrt

So, it's not a missing configuration but rather a design choice. Switching at CPU level is not very efficient on these SoCs so you'd prefer to do routing instead.