On Variants without a PHY (e.g. PicoCoreMX8MM-V1), are these signals routed directly to the respective connector pins with matching trace lengths?
-> Yes
Is there something important we have to consider when connecting an external switch/phy?
-> check voltages for the pins, some PHYs have different voltages for the MDIO interface and Reset/Interrupt
Do you have any reference designs or experience for connecting an external phy on the carrier board to a PicoCore?
-> the DDR3L variants of the PicoCore8MM have a 2 port switch on the board
Is it possible to run full speed a.k.a. 1Gbit this way?
-> Yes
Regarding the clock delay of 1.5-2ns needed by RGMII, this can either be done by the phy/mac devices itself (RGMII-ID) or in HW via trace lengths. I would guess the first method (RGMII-ID) is the simpler and more common one, correct?
-> Correct
Is there a big difference in connecting a device via RMII compared to RGMII, apart from the different signals? So in terms of trace lengths, delays, etc.
-> RMII is not critical, the clock is only 50MHz compared to 125MHz in RGMII. We have not tested RMII operation on the PicoCore 8MM.