Hello Paul
Many and profuse thanks for this thread; it has taken over as my morning first read.
I've been concentrating on modular power sections, separate from a digitally generated sine wave control section, for a 3-phase, brushless, AC motor. I've found some things that may help you, at least with the hardware end.
1. Instead of tin plating, consider silver plating. Here's the system I'm considering, "Cool-Amp" for plating the buss bars and PCB, and "Conducto-Lube" for an electrical/thermal interface. No heating, soldering, delaminating, alignment or parts placement errors. Not as cheap as tin plate and solder, but if parts can be replaced, reused or reclaimed with no more than a couple of hand tools, it looks good to me.
[I just did a quick ballpark (using dimensions from the rear of my mind) for the cost to silver your controller, PCB, buss bars and spreader: ~$3.75 to plate, ~$2.10 to thermal compound; not outrageous.] Check the spec sheets on each for coverage and prices.
Cool-Amp Conducto-Lube Co.
2. When it comes to cutting, drilling, etc. any kind of metal, I prefer "TapMagic" cutting fluid. Try a machine tool supply shop. "TapMagic Aluminum" is also good for cutting/drilling some plastics.
Tap Magic Metal Cutting Fluids
Also, in a photo I noticed the point of the drill you're using. It doesn't look like one I would choose for copper (or aluminum) and here's why. Do a search for this link instead of clicking on it here. In Firefox, it's the first entry on the first page.
Choose The Best Drill Point Geometry: MMSOnline.com
3. If the copper thickness on the PCB is a major concern; consider electroplating copper onto whatever raw PCB stock you can get your hands on, up to any thickness you want. You already have a copper substrate, just rough it up, scrub it clean (grease free), dip in copper sulfate and apply a low voltage. We did this in junior high decades ago. .010" or even .100" would be easy. You could even make multi-level (not layer) copper PCB. You'll need cross-section area calculations for the current to be carried by a particular trace.
This is fun.
John