TO-247 High Voltage pin clearance
I have designed a few boards like this in the past, and the first thing that I would do, if possible, was to stagger the middle pin inwards into the PCB, in order to increase the clearance to the outer pins. This is not always possible, so the next step is to cut slots between the pads.

This article explains quite well the issue of creepage

One thing that I find concerning is the mounting hole so close to the IGBTs, have you taken the isolation from that into consideration?