One way to be sure that your user's information is preserved after their account is deleted is to use a backup solution. Such solutions are readily available, and can back up mail and files for as little as $5 per user per month. Better yet, they are typically administered through a handy web interface, not PowerShell.
The best thing about having a backup solution is that it will typically cover more types of data than other approaches. Specifically, in addition to Exchange Online, these solutions typically also back up data in OneDrive for Business and SharePoint. If there is a downside to choosing this option, it would be that you may need to keep paying for the user's license for as long as you need ...
