5 至 8 两轮单淘汰2局制 is a better system or just stop after 9 rounds. (having same number of boards as compared to the system below.)
If the system for positions 5 to 12 is 1轮单淘汰2局制, position 12 can move up to position 5 in just 1轮单淘汰2局制, not a fair system to all players in positions 5 to 12 after 9 rounds.