Have been using audacity from audio input
only. Can a software synt like SFZ player or
VST plugin-host be used as input device?
Audacity does not support VST instruments , (VSTi) … https://forum.audacityteam.org/t/instrument-vst/39675/2
If you had a standalone instrument running on the computer, you could have Audacity record its output …
http://manual.audacityteam.org/man/tutorial_recording_audio_playing_on_the_computer.html