From what I can tell you're pretty much right. A linux bridge cannot possibly be as efficient or speedy as a dedicated switch asic. OpenWRT has support for a few different hardware switch kernel APIs, but you can't exactly buy one of those on a PCIe card and I've never seen one of those N100-class boards with one instead of a set of i226 ethernet controllers taking most of the PCIe lanes.

Mikrotik sells the CCR2004-1G-2XS-PCIe, which is a fascinating device:

https://mikrotik.com/product/ccr2004_1g_2xs_pcie

It is a full Mikrotik router stripped down to just a board and hung off a PCIe interface. Iirc by default it exposes a virtual gigabit interface to the host and otherwise acts exactly like a CCR2004 running RouterOS.

Doesn't really buy you anything vs a RB5009 unless you can use the pair of 25Gbps ports, but it sure is neat.