If you really want to extract mass from a star, just wait long enough, or find one ready to release a suitable amount. There are plenty of options from CMAs to flares to UV Ceti style flare stars, to deflagrating white dwarfs, novas, and supernovas. Let the star's nuclear energy do the "heavy lifting", eliminating all the problems catalogued before the "... plasma jets hundreds or thousands of astronomical units long ... [t]he details of extracting useful materials from [that has] not been extensively explored".
(The linked article at the top discusses relativistic jets driven by million-solar-mass and larger black holes. The jets are likely to be more constant and/or more predictable than natural mass outflows from stars, but extracting and storing energy from that is even less "extensively explored". An option in between is to use the natural mass flow from a companion star onto a compact star (white dwarf, neutron star, black hole) by sticking the moral equivalent of a water wheel into the stream. It's just a (compared to water hot & sparse) fluid flowing downhill, and there are plenty of examples in our galaxy.)
https://en.wikipedia.org/wiki/Star_lifting
Even without considering what to do with the resultant mass star lifting is just good stellar husbandry for a civilization looking to last.