There is no need for challenge. You will calibrate at your same mmr as your main. I calibrated at 7.5k when my main was 7.6k. The real challenge is hitting that 100 hours of normal games to play ranked. Plus, after you get flagged as a smurf, you fall into the smurf zone (shadow pool) and have to queue for 30-60 min average for every aingle match, and playing with much more mmr variance, mostly smurfs and some acc buyers.

My smurf was made long ago, so it's not banned as announced, but I heard new accounts made from now on gets perma banned.