Which ST board for prototyping STM32F415RG ?

You're using STM32F415RGT. So go to the STM32F415RG Product Page:

https://www.st.com/en/microcontrollers-microprocessors/stm32f415rg.html

Look on the 'Tools & Software' tab:

https://www.st.com/en/microcontrollers-microprocessors/stm32f415rg.html#tools-software

And look under 'Product evaluation tools' for available boards.

The STM32F4DISCOVERY is listed - it uses an STM32F407VG.

Downside of this board is that you have to manually connect the ST-Link's Virtual COM Port (VCP).

Alternatively, go to the Nucleo Boards product selector:

https://www.st.com/en/evaluation-tools/stm32-nucleo-boards/products.html

You can search by the microcontroller used; eg,

See which one most closely matches the feature set that you'll actually be using ...