There doesn't seem to be any way to use variable fonts in scripts. There is an undocumented property TextFont.axisVector, but it is read-only. So there is no way to adjust the axes of a variable font via script, even though Illustrator has supported variable fonts for many years now.

Relevant thread:
https://community.adobe.com/t5/illustrator-discussions/scripting-variable-fonts-in-illustrator/td-p/9834514