Didn't even think about using any of the Decos as actual routers. My bad.
My devices are all setup as dumb APs with wireless and wired connections bridged. I mainly use them for multiple SSIDs that lead to multiple VLANs. I don't even have a WAN interface anymore and am not using the firewall.
So yes, definitely activate Software flow offloading in Network->Firewall if you don't need SQM. That should give you speeds comparable to simply bridging the networks.
No clue if Hardware flow offloading works though. This suggests it doesn't: Hardware flow offloading/Hardware NAT? - #6 by slh