I just finished updating Sandeep's pico-rmii-ethernet library to enable line-rate Ethernet transmit and receive. The new library is available in Github: https://github.com/rscott2049/pico-rmii-ethernet_nce/tree/main.

From the readme:

This library uses DMA driven ring buffers for both transmit and receive. The transmit side is entirely DMA driven, while the receive side uses a per-packet interrupt to finalize a received packet. Performance does vary with system clock speed, and the memory region the executable is placed:

The MDIO interface was changed from polled to interrupt driven, thus freeing up more processor time for packet processing.