Using a thick spacer is a bad idea.. But a shim has never been an issue & in rare instances the OE manufacturers used them.....
If the rim was super close to the caliper before new pads would push the outer face of the caliper closer to the backside of the wheel so a shim could be your solution...
If the rim was super close to the caliper before new pads would push the outer face of the caliper closer to the backside of the wheel so a shim could be your solution...