Can I under inflate bike tires to reach a 15 percent drop?
You can try to lower the pressure further, but 2.8bar (40PSI) is already quite low for a 38mm trekking tire. Expect snakebites (pinch flats), increased rolling resistance and cracked sidewalls. There is no hard limit and if your rims are wide enough, handling shouldn’t suffer too much.

You could upgrade to wider tires if your rims, brakes and frame allow for it.

More flexible tires (e.g. cyclocross tires) would be a bit more comfortable and better at handling low pressure. However, they are generally less puncture resistant.

On a sidenote: A more relaxed and active riding style helps a lot. Try to avoid potholes or at least get out of the saddle!