The La Bellas
@cavemanbass mentions are not flats. They're larger-gauged than, but not very different in construction from the original D'Addarios = nylon-core silver-plated copper (as opposed to phosphor bronze) roundwounds:
Taylor GS Mini-e Bass (&ff.)
Here's a relatively new nylon-core, metal (chrome-nickel) flatwound option by Pyramid:
https://www.thomannmusic.com/pyramid_235_acoustic_bass_gs_miniset.htm
[There is also one by Galli (UXB910C, originally developed with Kala and also sold under their brand as KA-BASS-4FW)
Galli KA-BASS-4FW Chrome Steel Flat Wound Ukulele Bass, 23 scale
but it has significantly larger gauges, and probably a good deal more tension.]
A caveat: installing the Pyramids (and the Gallis) would entail reworking the slots of the peculiar bridge pins the Taylor sports, in order for them to accommodate the larger bulk of the silked ends of the flats. Or ordering an extra set of pins from Taylor, so as to keep one intact:
Flat wounds on a Taylor GS Mini-e Bass?
(It is possible not all pins would need the above treatment: for one or three, just shaving the silk may be all that's needed - perhaps not even that, as far as the G.)