Electronic – Attenuation curve of the twisted pair cable

Its actually a fairly complicated topic, I don't know of any "rule of thumb" calculation but there is an article from EDN here on transmission line parameters of twisted pair cables including the effects of pitch (twist rate). The twist rate will change the effective dielectric constant between the 2 conductors which does impact impedance and dielectric losses vs frequency.

There are also other things going on. For instance not many people know that the 4 twisted pairs in an ethernet cable all have slightly different twist rates. This prevents the same conductors from constantly and periodically being next to each other in the cable which would cause increased non-common-mode interference.

Yes, there is method to minimize crosstalk. Use very low speed and 1 turn pass-through transformer for every pair. Small ferrite bead with 2 wires = 4 ends. This will decouple each individual pair. On the picture below the ring is horizontal, wires enter ring from top and bottom.

copper wire A ---\ /--------\ /----------\  /----- A'
                  X  twists  X         [======] ferrite bead does "untwist"
copper wire A'---/ \--------/ \----------/  \----- A