With thinner substrate to gnd plane, you can achieve low impedance with thinner tracks and lower loss with less material in between.
But beware the dimensions become more critical with smaller size and electrical testing is recommended to validate Er with TDR supplier tests, while effective Er reduces with rising frequency in this range.
Z only depends on the surface ratios of the OD/ID for a given Er, and not so much the size.
However making a thin laminate makes it too flexible so a stiffener is needed underneath in some cases like bulk FR4 and surface roughness becomes more sensitive.
We used to make them in FPC this way for a flex cable to reduce flex but can be shaped.