I have to admit that I have no reliable information to share. However as far as I understand, software offloading should work with SQM as it tries to avoid other parts of the network stack higher up than mere qdiscs like sqm uses. Hardware flow offloading however will hide all packets from sqm, so the hardware offloading engine needs to offer its own qdiscs, like the NSS stuff on e.g. r7800 as far as I understand.
The pages from luci-app-sqm? These are independent from sqm-scripts for some time now, so anybody having reliable information could create a PR to get well-tested/well-researched changed in.
As I said, I have never bothered to test any offload. But it seems that my current router offers software flow offloading so I could go and test it... Maybe I will get around to do that later this year...