For insert effects create an insert routing send/return pair using FW cards input/output. For send effects route AUX to FW card output and FW card's input to one spare channel. Then just inplement correct routings in your software.
But remember. You don't have latency compensation with this approach. Everything routed through your computer will be delayed by few (maybe few dozen) milliseconds.
Maybe there is a delay setting in the X48? If you measure the delay the DAW gives you, you could get around with a fixed delay on all X48 tracks? Or maybe this is simply done by locking the DAW to MTC of the desk?