Any good ideas for that first stage are very welcome. I've looked at all kinds of mechanism (belts, pickers, vibrating drums) and none so far have the required speed, regularity and are quiet enough to be used in a home.
A combination of pins and holes will get you roughly sized items.
A vibrating table with lots of W snapped channels underneath is going to help too.
A belt with "scoops" is a great way to feed out of your initial hopper, if you drop it onto a perpendicular belt (one with W grooves) your going to get a good initial separation more naturally.
Air knives and puffers are your friend as well. Since you already have a detection system running it twice might be of benefit to you, just puff off anything that you can't sort as a single item.
I don't think "quite" and "lego" really go together so no happy solution for you there other than sound isolation.
Massive hopper (non-tapered if the tapering causes jams), feeding into slowly rotating (60 rpm) spiral-inner drum, falling onto very fast moving (10 feet per second) belt.
Parts get spread into a ~2 inch thick layer inside the drum, and then falling (at least a foot) onto the fast moving belt will then likely separate them further. For parts that end up entangled, end up on the belt too close together, or are unrecognisable in the orientation they fall, simply redirect them back into the top of the hopper to be resorted.
Even a 50% successful sort rate should be good enough.
I think that a couple of tables, cones, and dividers (to create lanes of pieces) could do a reasonable job of turning a heap of pieces into a steady stream of individual pieces.