There’s many patents out there, and historical instruments, but I’m not sure going back to them would offer many improvements. Borel clearly researched widely before developing the Accordina and the Clavietta in the 50s. Both masterful instruments, but both also flawed (in therms of volume, tone and note release ‘click’).
And I haven’t actually heard Wheatstone’s Symphonium, but I imagine that it woudn’t sound as good as a modern melodica. Hohner, Yamaha and Suzuki have developed remarkable instruments, as have Tononi and Borel etc. I think wer’e very nearly there. It’s just fine tuning that’s needed. In particular in the area of reed development
But fine tuning is costly and can take many years. Maybe we should start a research foundation?