For Years or War, rebirth a 2nd char, don’t need to level up.
Use the cube to farm a set that can be worn by the main char.
Give the set to main char to do GR55.
After the conquest, delete the 2nd char.
Recreate 2nd char as non-seasonal.
Claim the items from the mail.
Re-level up to 70.
This is what I did earlier.
Lv 1 monk got 5 Sunwuko pieces using the cube. Each pc took 1-15 gambles. DH (main char) took 7:30 to complete GR55. Recreate monk as non-seasonal and re-level up to 70 took 20 minutes.
The whole process took less than 1 hour.
(oops, I used a wrong gem.)