I hadn't pictured it clearly in my head but i was thinking that as there was no IGBT to the north, that area could be used. Thinking that the clearance between the heat sink and the bottom side of the copper ring would be enough to allow the control circuits to utilise the space to the north right to the edge of the heat sink if required.
However i can see there would probably be issues with the control circuits getting too close to the copper rings even if the PCB stepped down to a lower level closer to the heat sink.
What about if there were two separate boards one above the other, the lower board carrying the control circuits and the upper board just the drivers? The lower board almost touching the heat sink and the upper board at the correct level to solder directly to the driver pins on the IGBTs. That way the circuits would all remain within the confines of the inner copper ring.
Actually i should probably stop making suggestions. You have put an enormous amount of effort into the existing design and what i am suggesting is looking more and more like a total redesign.
Maybe best to go with what already works. Sorry to waste your time answering all these questions. I didn't expect it to snowball so much but i suppose that's how it goes sometimes. One change requires two more changes which require two more changes each and hey presto the whole design has changed. Maybe it should just be something to mull over until hardware version 2 is being designed.
I get people asking me things like this all the time in software development land. "We just want one small change, won't take you long" and it leads to a total rewrite and months of work.