Your 400+ rounds are not relevant. Your average resets each time you advance to the next tier. Then you need to play a certain number of Ranked Rounds from there to saturation, and you also need to reduce your average to the required target to advance again.
You have played exactly 36 Ranked Rounds now as a TM (today with Ave = 68.81).
Saturation will be reached at 40, so you are almost there.
After that you need to work your way towards the target of Average 61.000 or lower, which will take some time seeing as out of your 36 rounds as TM, only 1 of those rounds is at 61 or under.
Good Luck on the journey
Happy Hacking
Neil (The Hakman)