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.