So after one does teshuva, they don't have to experience bad things and sufferings as punishments/payback for their past mistakes (from before they did teshuva?) at all? like they are as if perfect and do not have a bunch of spiritual debts to payback?

Correct, but that doesn't mean that their life will be simplistic and without challenges or suffering! Not all challenges or "bad" experiences, are punishment for something we did wrong! It's childish to think that all bitter tastes are medicine or healthy, and all challenges are punishment. There has never been anyone, even the greatest of righteous people, who have an easy life. Wealth has challenges as does poverty, etc. etc. BTW, there is also no such thing as a perfect person, as Rav Kook says, "The strive for perfection is the most perfect a person can be!" (but again, that has nothing to do with their challenges). The world is complex and the challenges are here to help us grow, as Rav Soloveichik says, "The goal is to destine one's fate".