You need to get a proper MIDI master keyboard. Or,
if you *really*
want to use a PC keyboard, replace the keyboard MCU with one of
these boards they use for interfacing arcade joysticks and buttons
with the keyboard port or USB.
(Actually, you'd have to replace pretty much everything, as it's not
really the MCU, but the keyboard switch matrix that is at fault - and
that usually concists of three layers of thick plastic film these
days, so it's hard, if at all possible to modify.)
Hmmmmmmm... well, I think you've got me pretty much convinced there are
some limitations I've got to live with using the keyboard for sound :)
But I see you like to push the limits and I think that's really great!
Carlo