A few things I want to mention. OP, you mentioned PCB a few times, and the only TKL PCB we can get are phantoms, which I have no ties to. bphiphany designed it and controls the production of them. Teensy you can just buy on your own.
The plates however, I can help with those. I designed and ran the GB for all universal TKL plates, and all phantom R2 plates.
Cost between raw material is not much (only a few bucks). Majority of the cost in the plate is the laser machine time and programming fees. I had batches of aluminum and stainless steel plates before, and I personally like the steel one better for the weight and rigidity. Quality of the cuts on aluminum is a tad lower because they're more prone to local warping from high temps. Stainless steel is more resistant to temperature warping, so the edges turn out a little sharper and cleaner. As for color, anodizing adds a big chunk to cost and is only available for aluminum. Powder coating is also an option, but costs even more than anodizing, but it can be done for both aluminum or steel.
As for sourcing manufacturers, you'll have to source a company that has a laser cutter with at least 3kw with tolerances of 0.002" or better, otherwise switch and stabilizer holes will be too loose. I sourced about 15 different local places around me, price and quality varies between different companies/machines, so prototype is a must. You might be able to source cheaper places that are further, but shipping these heavy plates around is not cheap (hence why I did everything local, and picked them up myself).
I can help you get these made, or give you contact info for the local company. But just letting you know ahead of time that they do not deal with individuals, and you must send official blueprints (not just a raw DXF file) before they go through quoting process. If you want to do it through my account, then it'll be a few weeks before I have time to run another batch.
I have some left after shipping out the preorders, but there were more people that wanted plates than the numbers of extras I have. Not sure what the fairest way to sell them would be.