I believe Realtek RTL8125 (2.5 Gbps) has appeared in a few mainboards,
but I can only find two ethernet cards from a company called Speed Dragon.
And I don't think many retailers are offering them, so I wondered if anybody
else makes a NIC from RTL8125.

I know Aqnanita-based cards are out there (5 or 10 Gbps) cheaper than
Intel or Broadcom enterprise stuff. But the fast switches get expensive.

2.5G would suffice for me, with these blessings:
only need PCIEx1 slot
works with CAT5e cables
2.5G 8-port switch as little as US$300