Bela is not limited to C++ - you could also do this in CSound, PureData or Supercollider.
if you’re just doing a bit of midi->cv manipulation, this would be pretty trivial in Pure Data.
if you’ve done C before, this is also trivial to do in Bela… there are examples for midi and analog io, so its just a matter of hacking it together… you’d probably have the basics done in a 30 mins 
But it does raise a good point…
one of the reason I like Bela is their support is excellent, Guilo who monitors the forum, responds ridiculously quick, and can field not only simple beginner questions but also highly technical stuff… he really knows his stuff.
with something like arduino/teensy its not music focused, so I find you have to do a lot more of the legwork yourself.
so support is an important factor - imho
anyway… im sure any of the solutions you are looking at can be made to work,
Id just pick one - and go for it
have fun
Mark
one last thing to mention…
if you are outputting pitch (v/oct), calibration becomes very important… because tolerances in components of the hardware , mean it is never quite ‘exact’.
for the work Ive done with Bela Salt and the Soundplane, I manually calibrated my setup,
but Im in the process of looking to create a utility for Bela to calibrate both the input and outputs.
I guess this is only a concern for DIY type solutions.
Id assume ‘out of the box’ solutions, like Norns + Crow (?) , or Teletype already have their outputs calibrated for accurate pitch use. (though I would double check).
(though you can do this ‘calibration’ in your own software, i would not recommend it, its a bit of a pain in the neck, since every output needs to be calibrated individually - and may need a ‘complex’ calibration profile (curve) to work accurately)
