vaxoiva
The ssh file btw is not required, nor used. DietPi has SSH enabled by default, but should not play a role here?
The part
## USB0 Ethernet
allow-hotplug usb0
iface usb0 inet static
address 192.168.7.2
netmask 255.255.255.0
network 192.168.7.0
broadcast 192.168.7.255
gateway 192.168.7.1 ## ONLY if you have internet connection via usb0
should be added as separate drop-in config, e.g. /etc/network/interfaces.d/usb0
You can then disable Ethernet and WiFi within dietpi-config, so the other two interfaces are down.
AFAIK dtoverlay load required modules by default, so modules-load=dwc2,g_ether should not be required. Not sure about the g_ether option, at least for the overlay it is not supported:
Name: dwc2
Info: Selects the dwc2 USB controller driver
Load: dtoverlay=dwc2,<param>=<val>
Params: dr_mode Dual role mode: "host", "peripheral" or "otg"
g-rx-fifo-size Size of rx fifo size in gadget mode
g-np-tx-fifo-size Size of non-periodic tx fifo size in gadget
mode
You should check which module is actually enabled by the overlay itself (e.g. lsmod, perhaps it is different on current Raspbian/RPi firmware versions. /etc/modprobe.d/ could be used to add options/params to the module.
Apart from that we would need to compare the Raspbian setup then. DietPi is based on the official Raspbian image, but perhaps some additional APT package is missing. Explicite modules load as said should not be required with dtoverlays.