Hi,
USB high-speed (OTG on USB_DM2/DP2 pins) should be used to load either complete image or only up to u-Boot (to use Ethernet as proposed by @Community member ).
CubeProgrammer could be used. As not the fastest tool for a complete image, could be used for TF-A/u-Boot load before Ethernet use (I think from u-Boot, an USB Flash drive connected on USBH on USB_DM1/DP1 might be possible).
All these might need some small tailoring or scripting of our standard deliveries.
Alternatively, especially if you don't want to use Ethernet, our partner Elprotronic also provide a fast USB flasher: https://www.elprotronic.com/pages/gangflasher-st
For eMMC pin choices, i confirm the only restrictions are the 3 pins used by BootROM (and obviously the AFmux possibilities regarding other eMMC pins). See also AN5031 for more guidelines.
Regards,
Patrick