In systemd service file, how do I say "after USB is ready"?

I have not solved this before, but it seems like it could be a good fit for "path-based activation".

Instead of having an "After=" clause in your service file, you would create a .path file, as described in man systemd.path.

Find a suitable file under /dev/bus/usb or /sys/bus/usb, whose presence indicates that "USB is up". Then have systemd monitor the file path using the .path file you'll create. The .path file would then activate your .service file when the file exists.

What I would do is to create a systemd device unit using the an udev rule. E.g.: create /etc/udev/rules.d/20-usb-bus.rules with:

KERNEL=="usb[1-2]", TAG+="systemd"

At next boot (or udev rules reload) you will now have your system device unit:

# systemctl status dev-bus-usb-001-001.device
● dev-bus-usb-001-001.device - 2.0 root hub
...
# systemctl status dev-bus-usb-002-001.device
● dev-bus-usb-002-001.device - 3.0 root hub
...

You can now make your service start after USB bus is ready by adding:

[Unit]
...
After=dev-bus-usb-001-001.device dev-bus-usb-002-001.device

to your systemd service.