I'm among the first 6000 users of Instagram and my first name username was stolen a few years ago. Support for verified accounts acknowledged the issue, but couldn't do anything about it. This turn was an AI exploit, in my case was an outsourcing support 'exploit', where someone paid for my username to be manually changed and given to another user. There will always be a way to get access to accounts if human account…
Can you sue? I assume there is a financial motive with this crime.
Sue the anonymous person who stole your account and sold it to someone else, who is probably nowhere near your jurisdiction? Good luck.