Voltage spikes when driving DC motor with N-channel MOSFET
It seems to me that what you need is a voltage snubber across the MOSFET.  An easy way to do that is to simply connect a series capacitor + resistor across the MOSFET.  I'd guesstimate that a value of about 2.7 nF (about 3x capacitance of the MOSFET) and resistor of 100 \$\Omega\$ would be about right.

This ancient application note describes the various kinds of snubber circuits, including when and how to use them.  You might find some inspiration there.