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zyxel gs1900 series - can be low power, or go up to 48 ports, and even with POE (of course more power because, POE)

runs openwrt

There are realtek multigig/2.5GBASE-T switches that idle at 1W, or 2W for 8 port. Even the 4 port version here does 5W, and is just gigabit.

do they run openwrt?

no, even though I'm not sure they cannot.