I have some of these MCUDev "Black" F407VET6, ZET6, ZGT6 boards, each with 10/100 Ethernet MAC.
eg. https://github.com/mcauser/BLACK_F407VE
I saw a recent change which gave network.LAN() to other STM32 boards, NUCLEO_F767ZI, STM32F7DISC and STM32F769DISC.
https://github.com/micropython/micropython/pull/4541
Looks like it's tailored to use the LAN8742A PHY. Bundled chip on those boards, perhaps?
I couldn't find any LAN8742A modules for sale, but I did find a cheap LAN8720 one, with RMII interface.
https://www.aliexpress.com/item/32486319052.html
Could the LAN8720 module be used as a drop in replacement for the LAN8742A?
I don't know very much about these newfangled PHY chips, but it would be neat to add ethernet to my boards too.
https://www.microchip.com/wwwproducts/en/LAN8742A
https://www.microchip.com/wwwproducts/en/LAN8720A