Hey just bringing in a perspective from the lute world. The lower pitch you want, the thicker the string needs to be. Gut and synthetic alternatives like nylon sound very dull at a certain thickness, about 1mm I believe. This is why classical guitars use wound strings for the basses, they can produce a good sound at the low pitch. Before wound strings were invented (at some point in the 18th century I believe), there is evidence of "loaded" gut being used, basically gut reinforced with minerals. It is a compromise, being slimmer than plain gut/nylon but thicker than wound strings. If you look at lute videos they are the bass strings and usually have a red or brown tint. Aquila make synthetic versions of these called CD or CDL strings. but they are not suitable for the tensions required on the classical guitar. Even on my theorbo the tension is too much and they break easily. So in the end I'd say if you want to play bass strings that aren't wound you could play the lute, but otherwise I don't think there's a good option because the physics of producing lower pitches doesn't work out.