expression is mostly expected at CC2, but SWAM will learn which CC you send.
The CC should best describe the envelop. This envelop is contained within aftertouch. Since keyboards often dont have aftertouch, synth developers mostly swtich to CC control.
MG can generate aftertouch however… This aftertouch can be converted to a CC via a midimachine. Then SWAM will hear the CC ( play the guitar when SWAM says “move your expression controller”) and it should work.
The conversion midi machine is here: Aftertouch to CC
Now the original envelop is not that exciting for playing cello, unless you use an e-bow or real bow on your guitar. That is the reason that midi pedals or breathcontrollers are often used for this: the extra controller then provides the CC and is this not bound to the decaying string volume.