I had this exact same idea a couple years ago but it didn't go so well.  I bought what looks like the same exact DC-DC converter, same ratings, but the 5v output was really dirty.  It was bad enough to create interference patterns visible on the VGA output.  I didn't create a fancy PCB like you did and there was no -5v implementation (not needed) but I did solder all the connections really good and solid.  Basically I just added the converter in-line in between the ATX PSU and the 20-pin mobo connector, such that the converter was driving all the 5v lines on the 20-pin.  I powered the converter with some unused 12v lines from the PSU just like you did.  I got a couple Slot 1 boards to boot but as I said, the output was dirty.  I thought about trying again with some smoothing caps but never got around to it.

Given the very stable running and high conversion efficiency, it may be a perfect solution for the lhis long-lasting issue in retro builds.

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