An issue seen when using the Argon previously was that if using the ethernet featherWing it used pins from the two available SPI peripherals (SPI & SPI1) thus restricting the use of SPI due to D3 & D4 being required.
There was an non official bypass for this on the Argon but since it looks like P2 is going to face the same issue Is there any alternative pins to use for SPI1 or again we can have no official SPI peripheral with ethernet?
It will not be possible to use SPI1 and Ethernet at the same time. The control pins for Ethernet cannot be moved from their positions on the D pins that conflict with SPI1 at this time.
The workaround for the nRF52840 does not work on the P2 (RTL8721D). The nRF52 is pretty flexible about moving ports to other pins, but this is generally not possible on the RTL872x, or the STM32, for that matter, as ports do not connect through a pin multiplexer inside the MCU.