Hello,
I do not think you need to use Mbed OS 5 because of DISCO-F407VG. Since it is not supported in MbedOs 6+ but STm32F4 family is and you will need your own settings anyway.
- The PHY
There were a topic about something like that - EthernetInterface in mbed OS5 with lan8720 - Mbed OS - Arm Mbed OS support forum, in that discusion is a link to JojoS62/custom_targets: Custom target definitions for Mbed5 (github.com)- So maybe @JojoS can give you some hints.
- The USART2 is used for printf (debugging)
This settings can be override via mbed_app.json file when you inherit an existing target or you can set your own when you crate your own PinNames.h file.
BR, Jan