Unless I’m missing something here, pretty much any Intel nic released in the past decade should support tcp offload. I imagine the same is true for Broadcom and other vendors as well, but I don’t have something handy to check.
> Which end-user network cards that I can buy can do TCP offloading?
Intel's I210 controllers support offloading:
> Other performance-enhancing features include IPv4 and IPv6 checksum offload, TCP/UDP checksum offload, extended Tx descriptors for more offload capabilities, up to 256 KB TCP segmentation (TSO v2), header splitting, 40 KB packet buffer size, and 9.5 KB Jumbo Frame support.
Practically every on-board network adapter I've had for over a decade has had TCP offload support. Even the network adapter on my cheap $300 Walmart laptop has hardware TCP offload support.