Glad you've kept on with the project - it'l be worth it
I think you need to decide if you want to use a mechanical or an electronic paddle mechanism. If you want the latter, then you'd probably want a mechanical backup too, which would likely involve placing a lever on the tunnel (this needn't be too involved though - mine is a simple pivot which just bolts through the ali tunnel and pushes/pulls a rod, although I may strengthen.). So if you're aginst this, then I guess a mechanical paddle system is the only way to go. I haven't seen the STM paddles in the flesh, so I'm not sure exactly how much clearance you'd need, but even with my electronic paddle system, I had to pull the wheel out a bit from the dash. I'm sure a home-brew mechanical paddle system would be do-able, using your cable.
FYI, I'm using the Kliktronic electronic gearchange system, with a lever on the tunnel as a backup. For my paddles, I basically drew up something to suit my MOMO wheel and got the machine shop at work to do their magic. I then placed some microswitches and used some springs on short bolts to set the throw resistance and length. Took me ages to finish but well pleased with the result. I'm now remaking the gearchange linkage which links the mechanical and electronic devices to the gearbox...... A couple of piccies of my paddles here and here.
If you're considering the electronic route, Graeme Finlayson is currently selling his old Kliktronic & Translogic setup on EBAY - this is exactly the same system as I have (the Translogic is the ignition-cut part, for full-throttle upshifts 😬).
Bite me 😬