I wish you luck.
It's a bit late for you to learn this now, but the internet is forever. It's best to keep IRL stuff and your online presence completely separate. Never reuse a username that can be associated with your personal life.
<-- Something generic in the community you're posting in is suitable. You just can abandon it and start over if the SHTF.
It's not just MangaDex. You also can't trust the big tech companies to ever voluntarily give up the goldmine of personal data they've gathered about you. In some countries, you might have a right to be forgotten or right of erasure but you'll probably have to go through a tedious legal process and divulge even more personal information if you are to attempt to exercise such right and you'll never know if your data was actually erased or just moved to the secret folder where it will still be sold to data brokers and might still get leaked in the future.
Practice the appropriate level of OPSEC from day 1. It's not like you have to pull out all the stops for a MangaDex profile, but using a different username, picture, and e-mail address than your personal life is a good step to protect your privacy.
