You cannot make your users waive their legal rights for no good reason.
It's still a good idea to state in the ToS that users are not allowed to submit PII in certain forms. Such submissions can still happen of course, but having a "no private data allowed" rule gives you a good reason to remove the problematic content without upsetting your users.
"Letting anyone delete their own content" doesn't solve the problem, user A could use your service to post B's personal data, and you'll still have to deal with B's complaint.
"Letting anyone delete any content" is so prone to abuse that it'll be probably easier to deal with a GDPR complaint once in a while, than to explain to users why their data is gone.
Auto-deleting the content after a few days will be a much cleaner strategy with the same effect of practically avoiding GDPR complaints.
And BTW, you actually want to collect some private data about your users if you host their data, even for a few days. If user A posts B's credit card info, it will not be sufficient for B to have it removed. If B's information was used to steal their money, and B finds this information on your site, you'd better have A's PII when the police shows up.