Best way to replace DSL cable by CAT6 cable, to decrease resistances and improve connection.
Use 1 pair only.
Connect every 2 pairs together.
Connect White wires together and Colored wires together.
Note: Every pair is twisted.
The DSL line runs for hundreds or even thousands of meters on plain telephone cable (Cat-3ish). A few meters of Cat-6 will change nothing to improve signal integrity.
Just use any pair. Don't interconnect pairs.
Note that your middle drawing will degrade the signal significantly due to not using the twisted pairs. The bottom drawing improves nothing but might increase echos and distortions substantially.