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In terms of power usage, the best solution would probably be a passive Ethernet hub.

You can find crummy little 100mb switches on Aliexpress that actually power from micro USB. They're almost but not quite as bad as you think they'll be but for only $8...

https://www.aliexpress.com/item/33047436686.html

I tried one of these and I just found it was constantly drawing out 100mA+ of current. They're really not made with portable efficiency inside.

In the end, I just found that portable wifi solutions to be more power efficient.

OP could use something like a LAN9354 Ethernet switch and build their own switch. Probably needs a third PHY chip to interface with the Raspberry Pi as the Pi doesn't expose raw MII to attach to the LAN9354.